First attempt at doing this. Need more practise with the lighting and the setup. Have been waiting for the fish to die for years to get my hands on the tank to have a go :)
I had the camera pointing straight at the tank on a tripod with a cable release and my SB600 on a lightstand 90 degrees to my left pointing downwards at a 45 degree angle on 1/8th power.
Took a load of shots. I kept pressing the shutter release too soon. Ended up with water all over the kitchen table, chairs and floor.