This is a bit of a specialist area of photography, and I am sure the majority of people here will never have contemplated, let alone tried this. So thats probably the reason for no replies earlier.
With the narrow depth of field when using macro lenses or macro extension tubes, you would probably struggle keeping focus on anything which moves more than 1-2mm in the time you compose and click the shutter. Even if you turn off the pump(s), the fish are still going to be moving in 3 dimensions.
I'm guessing your tank will be 30 - 60cm from front to back, therefore, a relatively short close focussing distance would be handy as it would allow you to get closer to the subject, though macro would probably be more hastle than its worth to begin with. shooting through glass at 90 degrees can give lots of refrections, so aim to shoot at 30ish degrees and make sure theres no bright lights that are going to reflect towards you. If your lens can focus close enough, a flexible rubber lens hood against the glass may minimise reflections still further, even at 90'.
Oh and welcome