The flash technique I used was simply folding a piece of white kitchen towel/paper so it was about 4 layers thick (I played about taking test shots to get it looking good). I then used an elastic band to hold it in place over the flash Diffuses the flash so you don't get reflections too much. Oh, and I was shooting in a pitch black room too! Just the fish tank lights on.
Because the room was dark I think the fish were more relaxed as they couldn't see me, so they were willing to stay still longer