When I take my photos, I make sure that the camera can't see the gap between the surface of the water and the light (easy for me I just fill the tank past where the hood starts).
This stop the camera over exposing from the aquarium lights.
I have the room pitch black. so there is no reflection on the glass.
I DONT use the flash, but play around with the auto set white balance function on the camera normally appear as "day light, cloudy, auto etc etc
and if you can control shutter speed, turn it up, this way if the fish moves it wont be blurry.
And if your tank is bright enough turn down the ISO on the camera.
Getting the fish in focus and not blurred is most important, all the colour can be fixed in photoshop :lol: