As with fishkeeping, there are many ways to achieve any given result. but upping the ISO, will only lead to lower quality pictures. addressing the light, in the tank, is a much better way of dealing with the problem.
keeping the ISO low, will allow you to crop pictures, like below, and still keep good quality.
just occasionally there is no problem using flash, though it helps if you are used to Photoshop or the like. as in the past. its often the work done after the shot is taken that makes the picture.
even low shutter speeds can allow some good pictures. but a real understanding of how "photography, actually works helps. it is still true to say, no matter how much you spent on your DSLR. it can do nothing, except autofocus, that cameras from 40 years ago could. and DSLR's are still to achieve, even close to the outright quality, of those same 40 year old cameras.
in the end, whatever camera you have, practice and practice. when you finally, fully, understand the one you have, its time to change. spending money on DSLR's is fun, but not an answer to improving your pictures. that only comes when you understand how photography works. and no amount of "digital" help, can do that.