Fish and other subjects in tanks can be a great problem to 
photograph.  There are a number of problems other than the usual
photographic ones of depth of field and exposure.

Depth  of  field means a small aperture will be  required  to 
get enough sharp. This then tends to indicate flash will be
needed for both power of illumination and the correct colour
balance.

Fish  are also a greater problem than animals in air as the 
water adds both distortion and absorbs lots of light.

In this case,  the fish was stationary and close to the tank
front - reducing the problems.  The glass must be clean inside
and  out, and, if not, this should be done before the session
starts.

The  camera should be placed with it`s back parallel to the 
glass so that it is looking straight through.  This reduces 
distortion. Care should be taken to prevent reflections on the
glass where the lens will see them - windows behind the camera
for example.

A flash,  attached to the camera with an extension lead,  was
then placed close to the subject,  but in a position so that
light hitting the subject had passed through the glass at 45
degrees.  This means that any none image forming reflected light
will pass safely away from the camera lens, which is at 90
degrees to the glass.






Half the power of the flash will be lost due to reflection. 
Exposure  will  be largely by estimation and by shooting a 
number  of pictures  at  different values.  Cameras with off the 
film  plane flash  metering will make a reasonable attempt at
automatic  exposure,  and the Canon T90,  with it`s spot flash
metering,  usually gets it right.

I`ve fount that,  for mono, Ilford XP1/Xp2 helps,  as it has
great exposure latitude.