and _Actinodiscus sharonii_) that are about 1/4 inch when they are closed, open to about 3 1/2 to 4 inches when fully open under metal halide 5500 K Coralife* bulbs.
Metal halide lights, used properly, with UV shields, if so recommended, do not harm corals.
The sun gives off much more UV than even the strongest metal halides can, and beats on the coral reef day in, and day out, granted with the water as a partial shield.
Actinic light will, often, give the tank an eerie look, especially if you use several strong ones.
Metal halide lights can be expensive, especially if you need several of them.