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                           FAQ: Aquarium Filtration

contributed by Bruce Hallman

Summary

   This article describes how filtration can help ensure a healthy
   aquarium. The first half describes what filters are, and how they
   work. The second half evaluates the different types of filters.

Copyright

   The FAQs owe their existence to the contributors of the net, and as
   such it belongs to the readers of rec.aquaria and alt.aquaria.
   Articles with attributions are copyrighted by their original authors.
   Copies of the FAQs can be made freely, as long as it is distributed at
   no charge, and the disclaimers and the copyright notice are included.

Table of Contents

    1. Why you need filtration and how it works
          + Introduction
          + Water changes
          + Biological filtration
          + Mechanical filtration
          + Chemical filtration
    2. Filter types
          + Corner
          + Undergravel
          + Sponge
          + Power filter
          + Canister and Submersible
          + Wet/Dry (Trickle)
          + Protein skimmers
          + Fluidized bed
          + Denitrators
          + Algal Scrubbers
          + Chillers
          + Sterilization
    3. For More Information

1. Why you need Filtration

   Sometimes we forget that fish kept in an aquarium are confined to a
   very small quantity of water as compared to their natural habitats in
   the wild. In the wild, fish wastes are instantly diluted. But in an
   aquarium, waste products can quickly build up to toxic levels.

   These waste products include ammonia released from your fishes' gills,
   fish poop, and scraps of uneaten food. The food and the poop will also
   eventually decay, releasing ammonia. Even small amounts of ammonia
   will kill your fish.

   Obviously, the more sources of fish waste, the quicker and greater the
   ammonia problem. A small heavily-fed tank with lots of large fish will
   have much more ammonia than a large tank with one seldom-fed small
   fish. But for both these cases you need some form of aquarium
   filtration to control the toxic ammonia.

   Some aquarists try to control ammonia levels exclusively by changing
   the water. This is helpful, but impractical because of the frequency
   and size of the water changes required.

   Fortunately, there is an easier way! In fact, the world is full of
   bacteria that want nothing more than to consume the ammonia and
   convert it into less toxic substances. For many an aquarist, this
   process occurs without their knowledge or help. However, the smart
   aquarist will learn how to take advantage of this beneficial bacteria
   by maximizing its growth.

   When you start a new fish tank, colonies of beneficial bacteria have
   not yet had the chance to grow. For a period of several weeks this is
   hazardous to fish. You must gradually build up the source of ammonia
   (i.e., start with only one or two small fish) to allow time for the
   beneficial bacteria to grow. This is called ``cycling'' your tank.
   Read more about this in the BEGINNER FAQ.

   Remember that the bacteria break down the ammonia into substances
   (first nitrite, then eventually nitrate) that are merely less toxic,
   rather than non-toxic. Many fish can tolerate reasonably high levels
   of nitrates, but over time the nitrates will accumulate until they,
   too, become toxic. Also, because nitrate is a fertilizer, high nitrate
   levels can lead to excess algae growth.

  Water changes

   Although there are many ways to remove excess nitrate, the most
   effective way is to regularly change part of the water. This is one of
   the most neglected and important parts of aquarium maintenance!

   How often and how much you need to change depends a lot on the waste
   load in your tank, and the sensitivity of your fish. You don't want to
   change ALL of the water at any point in time because the change in
   water chemistry will be stressful to your fish. The best way to decide
   how often and how much to change your water is to monitor your water
   quality with water tests. As a minimum, if your tank is new, you
   should test for ammonia and perhaps nitrite. In established tanks you
   should monitor for nitrate accumulation. Read more about water tests
   in the TEST KIT SECTION of the BEGINNER FAQ. Water tests are the most
   reliable way to know how well your aquarium filtration works.

   For an average tank, you should change no more than one third of the
   water in 24 hours. Many aquarists with average aquariums change a
   quarter of the water every two weeks. Your aquarium is probably not
   average, and you really should measure nitrate levels to determine
   your water change schedule.

  Biological filtration

   Biological filtration is the term for fostering ammonia-neutralizing
   bacteria growth. It is so important to the health of your aquarium
   that we should look at how this process works more closely. (There are
   other types of wastes that can cause problems, but the regular partial
   water changes needed to control nitrates are typically enough to
   control other forms of waste as well.)

   Mother Nature provides several types of bacteria that break down
   ammonia into progressively less toxic compounds, nitrite and nitrate.
   These bacteria are not harmful and are quite abundant in nature. They
   are so common that we do not need to add them to our aquariums; nature
   does it for us.

   In the presence of ammonia and oxygen these bacteria will naturally
   multiply. The bacteria attach to the tank, rocks, gravel, and even
   tank decorations. Note that we have not yet said anything about a
   physical filter. This is because biofiltration bacteria require only
    1. A surface upon which to attach,
    2. ammonia for food, and
    3. oxygen-rich water.

   This sounds so simple; why do we need a physical filter?

   Actually, if you limit the amount of fish to what the natural
   biofiltration can handle, you do not need a physical filter.
   Unfortunately, you cannot support very many fish with only the natural
   biofiltration.

   In the last few decades, the hobby has seen many new types of
   biological filters invented which can vastly increase the capacity of
   the bacteria colony to provide biological filtration to your aquarium.
   In essence, all of these types of filters provide additional surface
   area for bacteria attachment and increase the available oxygen
   dissolved in the water.

  Mechanical filtration

   Remember that ammonia comes directly from the gills of your fish, but
   also from decaying fish poop and food scraps. If you can mechanically
   filter out the poop and the waste food before it gets a chance to
   decay, you can be a step ahead in the game. Not to mention that these
   wastes are ugly and can detract from the beauty and enjoyment of your
   aquarium.

   Simply stated, mechanical filtration is the straining of solid
   particles from the aquarium water. Mechanical filtration does not
   directly remove dissolved ammonia. Most common mechanical filter media
   do not remove microscopic bacteria and algae from the water. Neither
   will mechanical filtration remove any solids trapped by gravel,
   plants, or decorations.

   You will need another method to remove the solid wastes from the nooks
   and crannies of your aquarium. One of the easiest methods is to
   ``vacuum'' the gravel, etc., as part of your regular water change
   routine and everybody should do this. (Note that those marine
   aquariums which use ``live substrates'' are an exception.) Some people
   install circulation pumps, known as wave makers, to improve the chance
   of catching solid wastes in the mechanical filter.

   The four most popular mechanical filtration media are sponges, paper
   cartridges, loose and bonded floss media which are reusable to
   different degrees. Clean paper cartridges have the smallest openings
   and clean bonded floss has the largest openings. Clean sponges and
   clean loose floss fall somewhere between.

   A filter media with small openings catches finer particles, but clogs
   faster. Also, as a rule, a physically large filter area will clog more
   slowly than a small filter. As the filter media gets dirty it will
   trap smaller and smaller particles. At some point the media is so
   clogged that it will not pass water.

   SUMMARY: A good mechanical filter is one that traps enough solids to
   keep the water clear without plugging too often.

  Chemical Filtration

   Chemical filtration, in short, is the removal of dissolved wastes from
   aquarium water. Dissolved wastes exist in the water at a molecular
   level, and fall into two general categories, polar and nonpolar. The
   most common chemical filtration method involves filtering the water
   through gas activated carbon which works best on the nonpolar wastes
   (but also removes polar wastes). Another effective method is protein
   skimming, which removes polar wastes such as dissolved organics.

   Granular activated carbon (GAC) is manufactured from carbon, typically
   coal, heated in the presence of steam at very high heat. This process
   causes the carbon to develop huge numbers of tiny pores, which trap
   nonpolar wastes at the molecular levels by means of adsorption and ion
   exchange, and removes heavy metals and organic molecules, which are
   the source of undesirable colors and odors, through a process known as
   molecular sieving.

   The best GAC for filtering water is made from coal and is macroporous
   (having larger pores). A good macroporous activated carbon feels light
   (not dense) and fizzes and floats when initially wetted. GAC intended
   for removing wastes from air (such as odors) are commonly made from
   coconut shell and are microporous. Carbons for filtering air feel more
   dense.

   Some people (especially those with reef aquaria) are concerned about
   phosphate leaching from activated carbons. As a rule, buy only carbons
   made by reputable aquarium supply companies which have been acid
   washed during manufacture to minimize ash content. Carbons low in ash
   also help reduce the chance of undesirable pH shifts. Low ash carbons
   typically have lower phosphate leaching levels too.

   The phosphate in GAC stems from the fact that activated carbon is
   manufactured from coal, which was once living plant matter. All living
   matter is high in phosphates. The leaching of phosphate from GAC is
   known to be high initially and to decrease over time. This problem can
   be mitigated significantly by presoaking your activated carbon for a
   few weeks before use.

   Some people are concerned about GAC removing trace elements required
   by plants and invertebrates for healthy growth. Trace element
   depletion is a problem in planted aquaria and minireefs, with or
   without activated carbon. The potential benefits of activated carbon
   are great enough that on whole you will be better off using it. If
   trace element depletion is a worry, use a trace element supplement in
   conjunction with the activated carbon.

   GAC cannot be rejuvenated outside a laboratory, but fortunately, it is
   cheap enough to use liberally. Always wash your carbon before use to
   remove the dust that accumulates during shipment. Advice on how much
   to use vary, but smaller amounts changed more frequently seem to work
   best. You probably want to experiment, but 1/2 cup per 20 gallons
   water, changed monthly is a good starting point. In summary, activated
   carbon is an excellent, cheap and effective filtration method which is
   highly recommended for all aquaria.

   A variety of special chemical filtration media have been developed to
   remove specific chemicals. A common one is made from the zeolite clay
   (also used as some cat litters), and is marketed under such brand
   names as ``Ammo-Carb''. This media removes ammonia from water, and is
   good for short term use. The aquarist should be aware that if zeolite
   is used, especially when cycling a new aquarium, then the
   establishment of natural biological filtration will be delayed or
   disrupted.

   Protein skimmers are primarily used in saltwater aquaria, especially
   reefs. They have the remarkable ability to remove dissolved organic
   wastes before they decompose. The process involves taking advantage of
   the polar nature of the organic molecules, which are attracted to the
   surface of air bubbles injected into a column of water. The resultant
   foam is skimmed off and discarded.

2. FILTER TYPES

  The humble corner filter

   For decades, hobbyists have successfully kept fish healthy and happy
   through the use of the $2.49 corner filter. Typically, they are clear
   plastic boxes, which sit inside the tank. An air stone bubbles air
   through an air lift tube, which forces water through a bed of filter
   floss or other media. mechanically filtering the water. Colonies of
   bacteria build up on the media, providing excellent biological
   filtration. (It is important to change only some of the media at any
   given time! This way the bacteria does not get wiped out.) Nowadays
   people don't use corner filters as much because they're ugly, take up
   space in the tank, and require a bit more frequent maintenance than
   other filters. But you can't beat the price.

   Another use of the corner filter, that is not really matched by other
   filter types, is as an impromptu quarantine tank filter. If you have
   the need to set up a second tank on the quick, you can take some
   gravel from an established tank and put it in a corner filter, and
   immediately, you will have a functioning biological filter. This way
   you can turn a five gallon bucket into a quick and cheap
   hospital/quarantine tank on a moment's notice.

  Undergravel Filters

   Fish stores commonly sell undergravel filters (UGF's) to beginners in
   ``aquarium kits'' because they are cheap, and they work (for a while).
   UGF's work by slowly passing water through the bottom gravel, which
   sits on top of a perforated plate. The water can be pumped with an air
   lift, with bubbles air lifting the water in a vertical tube attached
   to the filter plate. Also, some people prefer the increased water flow
   achieved with submersible pumps, called powerheads, attached to the
   same lift tubes.

   UGF's make good biological filters, because the slow flow of water
   through the gravel fosters large colonies of beneficial bacteria which
   neutralize toxic ammonia. The hitch is, that UGF's are awful
   mechanical filters. Fish waste gets pulled out of sight into the
   gravel. Before you know it, the gravel clogs up. You then have a big
   mess and a health risk to your fish!

   A partial solution to this dilemma is to run the powerhead in reverse,
   sending water up through the gravel. This technique is known as
   Reverse-flow Undergravel Filtration (RUGF); conversion kits or special
   powerheads can be purchased to accomplish this. The intake of the
   powerhead is covered with a porous sponge which serves to "prefilter"
   out some of the waste that can clog the gravel. In actually practice,
   this helps, but is only a partial solution.

   If you choose to use an UGF/RUGF, you must regularly vacuum your
   gravel. Fish stores sell siphon hoses with a ``wide mouth gravel
   vacuum tube'' attachment that ``washes'' the gravel during your
   regular water changes. IF you clean your gravel regularly, and
   maintain a regular and frequent partial water routine, UGF's and
   RUGF's are an economical and effective aquarium filter in freshwater
   aquariums, and in lightly stocked saltwater fish-only aquariums.

  Sponge filters

   Sponge filters provide an efficient and cheap form of biological
   filtration. Water is forced through a porous foam, either by a
   powerhead, or air bubbling through an airlift tube. Water flowing
   though the sponge allows the growth of a colony of beneficial bacteria
   which neutralizes toxic ammonia.

   One style of sponge filter uses two sponges attached to one lift tube.
   These have the advantage that the sponges can be cleaned one at a
   time, reducing bacterial loss. Also, one of the sponges can be removed
   and transferred to a new tank, bringing with it a colony of beneficial
   bacteria, and thereby "jump starting" the cycling of a new tank. Some
   enlightened fish stores sell these double sponge filters to beginner
   customers when they sell a tank kit. They take one of the new sponges
   out of the "box" and swap it for a old established sponge in one of
   their tanks in their store which is carried home in a plastic bag.

  Power filters

   Most people agree that power filters are much easier to maintain and
   can be as economical as undergravel filters. There are many styles of
   power filters, but the most common hangs on the back of the tank. A
   siphon tube pulls water from the tank into the filter box and passes
   the water though a mechanical filter (typically a porous foam sponge).
   The sponge doubles as a biological filter. A internal pump then
   returns the filtered water into the aquarium. These power filters come
   in many sizes suited for small to large aquariums.

   The foam sponge can be easily inspected for clogging or removed for
   cleaning. You must clean the sponge regularly to remove the solid
   wastes before they decompose and dissolve back into the water. It is
   quite important that when you clean the porous foam that you do not
   kill the bacteria colony through the use of detergents, very hot or
   very cold water. A safe and easy way is to rinse the foam sponge in
   the bucket into which you have just drained some tank water during
   your regular water change routine.

   Power filters now come with all sorts of fancy ``features''. Most
   allow placement of a chemical filtering media, typically granular
   activated carbon, in the water path.

   Another development in the last few years is the ``wet-dry wheel''
   (called a biowheel by one manufacturer). The beneficial bacterial
   colonies that neutralize toxic ammonia require an oxygen rich
   environment to grow. The ``wet-dry wheel'' passes water over a water
   wheel device which sits outside (on the edge) of the aquarium. This
   rotating wheel maximizes available oxygen allowing a large bacteria
   colony to flourish. One drawback is that these wheels have been known
   to jam, so you need to check them frequently. Other than this minor
   point, the ``wet-dry wheel'' is an excellent method of providing
   vigorous biological filtration.

  The Canister filter

   Canister filters have some similarities with the ``hang on tank''
   style of power filters, but the essential difference is that canister
   filters are designed to provide more powerful mechanical filtration.
   Typically, the water is pumped, at moderate pressure through a filter
   material, such as glass wool, or a micron filter cartridge. Canister
   filters are especially useful in aquaria with large or numerous messy
   eaters that generate a lot of waste. For these filters to be effective
   they must be frequently cleaned, to avoid the decomposition of waste
   in the water stream.

   These filters usually sit on the floor below the tank, but also can
   hang on the tank, and in some designs even sit inside the tank, in
   which case they are called a ``submersible filter''. Some hobbyists
   attach a ``wet-dry wheel'' to the outflow of their canister to improve
   the biological filtration capacity of this type of filtration system.

  Wet/Dry Filters

   Also known as trickle filters, wet/dry filters work on the principle
   that the beneficial colonies of ammonia neutralizing bacteria grow
   best in the presence of well oxygenated water. By ``trickling'' water
   over unsubmerged plastic gizmos or other media, wet/dry filters
   provide a very large air/water surface area. They come in many shapes
   and sizes. The boom in successful saltwater aquariums in the 1980's
   can be attributed to the use of this filter type.

   Many things can used for the media, with the best providing great
   amounts of surface area, while at the same time having large openings
   to reduce the tendency to clog and ensure efficient gas exchange. The
   problem of clogging of the media can also be reduced by prefiltering
   the water with an efficient mechanical filter, and (when used) with a
   protein skimmer.

  Protein skimmers (aka Foam Fractionators)

   Protein skimmers were initially developed for use in industrial sewage
   treatment plants where they are also known by the term foam
   fractionator. Protein skimmers have the unique ability to remove
   dissolved organic wastes BEFORE they decompose! This is a neat trick
   which is accomplished by taking advantage of the fact that organic
   chemicals are attracted to the surfaces of bubbles which are passed in
   large numbers through a column of water. The foam is then ``skimmed''
   off the water, while at the same time removing the organic wastes. The
   foaming process only works in a water with high pH and salinity, and
   as a result skimmers are primarily for saltwater use.

   The protein skimmer is largely responsible for the boom in reef
   aquaria in the 1990's, due to the high water quality possible with
   this type of filtration. A current ``state of the art'' in reef
   systems is based upon the use of protein skimmers and live rock
   without the use of a wet/dry filter. This school of thought is known
   as the ``Berlin method''.

  Fluidized bed filters

   Very recently, some hobbyists have reported success with a new type of
   filter which uses a fluidized bed of sand. This filter is roughly
   similar in principle to the reverse flow undergravel filter, but with
   much higher water flow. The higher water flow keeps the sand clean of
   debris, while at the same time allowing the development of large and
   efficient colonies of beneficial bacteria. Reported problems include
   oxygen depletion and clogging.

  Denitrators

   Another specialized type of filter is designed to help in the control
   of the accumulation of nitrates, the end product of the neutralization
   of ammonia by the biological activity of bacteria. These fall into two
   categories, the anoxic bacterial, and the plant/algal scrubbers
   (discussed in the next section). It has been discovered that colonies
   of bacteria which grow in oxygen poor environments can be harnessed to
   biologically consume nitrate, and release harmless nitrogen gas. This
   method is achieved in one of two ways. The process was first developed
   in the 1980's through the use of a box system, coil, or porous foam
   block which allowed very slow transmission of nitrate-laden water.
   Inside the box/coil/foam, sugar was placed, and the slow passage of
   water quickly became anoxic. In these anoxic conditions, bacteria
   would grow and consume excess nitrate. Many aquarists have reported
   failure in their attempts at this type of filtration.

   More recently, hobbyists have developed similar anoxic conditions
   below plates at the bottom of their tanks buried in fine sand. In the
   saltwater systems, these sand beds are referred to as "live sand". In
   freshwater planted systems, fine grain substrates are allowed to
   develop anoxic zones which probably also have a denitrification
   capability.

   The Berlin Method of reef aquariums involves the use of large
   quantities of live rock harvested from tropical reefs. Aquarists
   report good nitrate control in live rock systems, which, though not
   well understood, probably involves the denitrification of the nitrates
   within the interior of the rocks. Another school of thought is that
   the heavy growths of calcareous algae on the live rocks in Berlin
   Method reef aquariums consume nitrate.

  Algal Scrubbers

   Algal scrubbers use live algae to do the ``filtration''. Water is run
   over a wire mesh in a trough under bright lights, where algae is
   encouraged to grow. The growth of the algae removes some pollutants
   from the water. This is a controversial form of filtration for reefs
   and large marine ecosystems invented by Dr. Adey at the Smithsonian.
   Some believe it is a complete filtration solution, others claim its
   use leads to poor water quality and algae growth in the tank as well.
   In freshwater planted aquariums vigorous plant growth has been
   observed to beneficially consume excess dissolved nitrates.

  Chillers

   While not really a filtration, saltwater aquarists occasionally have
   the need to lower the temperature of their aquarium water. The high
   light levels needed in reef aquaria involve a build up of excess heat.
   Use of a hood fan and removal of the ballast from the vicinity of the
   tank can also help. Submerged pumps are also a source of unwanted
   heat, and as a solution, reef aquarists favor the ``non-submerged''
   pumps due to the decreased transfer of heat to the water.

   A little recognized source of heat control is through the natural
   cooling effect of evaporation in wet dry filters, and through the flow
   of air over the surface of the aquarium. Nevertheless, additional
   cooling is often required, especially in warm climates.

   This is achieved through the use of "freon" style cooler units similar
   to home refrigerators. They either pass the water through a heat
   exchange unit, or pass coolant through a heat exchanger in the tank.
   Those chillers are expensive but not many people have had success in
   the "do it yourself" construction of chillers. (The "dorm" type of
   refrigerator is not powerful enough to be of use, just in case you
   were thinking about this.)

  Sterilization

   In especially sensitive aquaria, infections resulting from water born
   parasites, fungi, bacterium and vires can cause serious problems.
   Water sterilization is most beneficial for breeders (as it can help
   control infections of incubating eggs), for centralized multi-tank
   filtration (to control the spread of disease between tanks), and for
   delicate and/or costly setups such as large tanks and reef systems (as
   a safety measure). It is important to remember that a healthy aquarium
   depends on beneficial bacteria typically growing on media in your
   filter which neutralize ammonia. At most, your sterilizer can kill
   some water born pathogens, but total sterilization is not possible or
   desirable. Aquarists who practice prudent quarantine procedures for
   newly acquired fish generally do not need to sterilize.

   Two main types of sterilization are used, ozone injection and
   ultraviolet irradiation.:

    Ozone

   Ozone gas is highly reactive and is a powerful oxidizer of organic
   pollutants, including living pathogens. Another benefit of water
   treatment with ozone gas is that it systematically reduces dissolved
   organic compounds in the water stream which increases the reserve
   capacity of the water to oxidize organic waste throughout the
   aquarium. Ozone laden water also improves the ability of protein
   skimmers to generate foam which increases their overall performance.

   Prior to the discovery of the live rock/protein skimmer "Berlin
   Method" style of reef keeping, ozone injection was considered part of
   a "state of the art" filtration system, especially among Europeans in
   the 1980's. The trend of late is towards the more simple and natural
   Berlin Method. Though ozone use remains beneficial, it is being used
   less in recent years among reef keepers.

   Ozone gas is produced by devices which create a spark in dry air. As
   humidity drastically reduces the efficiency of ozone generators most
   aquarist choose to pretreat the air for the ozonizer with a
   dehumidifier. Ozone gas is highly corrosive, all elements (especially
   rubber) which can come in contact with ozone must be made from ozone
   safe materials (commonly silicone). Residual ozone can be efficiently
   stripped from air by passing the air through activated carbon. Ozone
   must not be allowed to enter your aquarium because it can kill your
   fish and invertebrates and/or damage the beneficial bacterial in your
   biological filter. Also, ozone gas is unsafe to breath and can cause
   irritation even in small concentrations.

    Ultraviolet Sterilizers

   High intensity ultraviolet light destroys the DNA in living cells and
   can be an effective means to control living pathogens. The most
   effective UV light is the high energy UV(C) light roughly at the
   wavelength of 250 Angstroms. To be effective, UV Sterilization (UVS)
   must expose the pathogens to high enough light intensity for a long
   enough period of time. Martin Moe cites 35,000 to 100,000 microwatts
   per second per square centimeter as the norm, which works out to
   roughly 10 to 25 gallons per hour per watt (or less for units not
   operating at peak efficiency).

   Common problems which can reduce efficiency and kill rate are:
    1. Allowing the water to flow too fast past the UV light.
    2. Light blockage due to a build up of salt deposits or bacterial
       slime on the bulb.
    3. Fading of the light due to age of the bulb (which typically have a
       six month life.)

   The same property of this light that kills germs can damage your eyes,
   and special care MUST BE TAKEN to avoid direct or indirect eye contact
   with this light. [This is especially serious because the damage occurs
   inside your eyes before you feel any pain. Too many people have
   already damaged their eyes in this way!] The UV(C) light does not
   penetrate water very well, so to be effective, UV Sterilizers commonly
   position the UV bulb close to the water which also can pose a risk of
   electrical shock should the bulb break, etc..

   There are three types of UV Sterilizers:
    1. Tray type. (Typically homemade) with UV bulbs suspended in a
       reflecting fixture over a shallow tray of slow flowing water.
       Benefits: easily cleaned, can be cheap, can be made large enough
       for commercial applications. Problems: safety risks to your eyes,
       too large and awkward for many home uses.
    2. Tube type, wet bulb. Tube types have the benefit of exposing all
       sides of the UV tube to water with no reflector. The water passes
       directly past the bulb which is mounted in a waterproof tube.
       Benefits: cheap, compact and effective. Problems: difficult to
       clean the slime accumulations from the bulb, safety risks due to
       electrical shock.
    3. Tube type, dry bulb. Similar to above, but the UV tube is
       surrounded by a quartz tube [glass blocks UV(C) light] insulating
       it from the water. These are more expensive and probably safer.
       Changing the light bulb is easier and dry bulb tube types can have
       a internal device to wipe slime from the quartz tube. Some of
       these types come with sensors to monitor the intensity of the
       light to let you know when to replace/clean the bulb. etc..

To learn more

   See the RESOURCE FAQ for several good books. A good reference work for
   aquarium filtration is Marine Aquarium Reference (Systems and
   Invertebrates) by Martin Moe