Thank you for the comments, my friends!
Mervin, The most of my shots I made with EF 50 mm/f2.5 Compact-Macro lens
+ Two External Flashes - Canon Speedlite 430EX situated over the tank(slave mod),
and 580EX like straight “fill” flash (slave mod), wires connected with the camera in “master” mod.
My usual settings:
Shutter Speed: 1/200 seconds
Aperture Value: f/14-22(dependent on the fish specific and fishes position)
ISO Speed Ratings: 100-200(dependent on the fish specific)