the tank light on, the room light off, so the out side of the tank is totally dark. dont use the flash. And if your camera has present white balances (auto, daylight, cloudy, fluorescent and incandescent) just flick through them till one looks the best. and i have my camera setting to twilight portrait
if there is a gap between top of the water and the lid of the tank, you may want to try cover it, my camera seems to lens flare along that gap
just took these on my phone to show the difference