What sort of parameters are you dealing with, you say 1/15th slowest shutter, ISO of 400-800, but what about f-stop?
those shots I linked to were all taken on f/2.8, wide open at 105mm like yours
ideally I'd like to close it a bit, but you can still see plently of detail
another idea might be to suspend a flash on a hotshoe adapter (cheap) above the tank aiming downwards to give you more light, but without any reflections
I havent tried it but it might work
my fish dont dart around too much so I can get away with 1/15 on a tripod with a remote release