I rarely use any lighting beyond the tank lights. The tank glass tends to glare too much. I have taken the flash off camera before and either put it right above the water line or right up against the tank. It produced marginal results

I have done better shooting in raw and matching the color temp of the metal halide bulbs(mine are 20k)

One of the hardest things to deal with is the glass of the tank. The reason I did these the other night is my husband scraped the coraline algae off(it is like purple cement) I actually had a clear shot. Not to mention you cannot angle the camera unless you want distortion. Has to be perpendicular

But, to expose properly I tend to bump the ISO open up all the way and get the shutter speed to at least 125. Everything in the tank moves, either the fish, polyps swaying or the tentacles.

Some people turn off their tank pumps so some of the movement stops.