One of my favorite fish photo models, my zonatum...the Blue Beast.  I set up the lights a little differently here.  Normally I will put two or three lights on top and one on the bottom.  On this shot I placed two flash units on the bottom.  The main difference was in the direction of the light.  Normally I would angle the flash, diffused, straight up.  This would give me a nice even lighting.

This time I placed one on the bottom and forward on the tank and directed the light to angle across the bottom from left to right.  The second flash unit I put farther back and angled straight up.  The effect I was trying to get is  more dramatic lighting effect...and in particular lighting the back bottom end of the fish.

The first shot the fish was JUST outside the limit of the top flash.

Better shot.  The additional flash on the bottom paid of by illuminating farther back on the fish, giving me a much deeper DOF.

Once again, heading out of the light.

The "Money Shot"!