Nice Fish pics.
Another way (other than the angle tip) to stop flash reflection if you have a dslr, would be to use an off camera flash with a sync cord. You can hold the flash over the top of the tank this way. Flash can tend to wash out the fish colours though, so maybe back off the flash power a bit and use a piece of white tissue paper as a diffuser. If you only have a point and shoot camera, try the angle method as also try to diffuse the flash again with tissue paper.