Sorry for the delay.
Since you aren't using flash, you want to optimize what you got.
1st, for a moving fish, you want a reasonably fast shutter speed. 60 works, but 100+ is better
if you can hit 200-400, awesome!...but unlikely with out flash
Depth of field...ie how "deep" is the focus range. I try and shoot with an aperature of ~5-8. much lower and you end up with too much of the fish out of focus
ISO: the higher the iso, the more noise, but the faster the shutter speed for a given aperature. I like to use the lowest possible, bus sometimes I have to shoot at iso 200 to gain some shutter speed
Exposure: Cameras tend to over expose aquarium pictures. This means you have needless long shutter speeds. I usually set mine at -0.7 or -0.1
Optimizing the above will get you crisper pictures. Hope my general nubmers work for you camera/light conditions.