What lenses are you using?
I've noticed my macro lenses get terrible distortion when shooting through the tank glass, particularly at any kind of angle other than dead on. (I mean you're dealing with a tiny DOF already with a macro, any distortion at all just makes it worse)
I've found that by 1) turning off all the lights in the room other than the tank's lights,
2) useing a smaller lense (not a macro or near focus telephoto)
3) using a higher fstop (creating a larger DOF)
4) using a tripod (pretty much a must when compensating for a tighter/higher fstop)
5) shooting at a perpendicular angle to the glass
all will help you get crisper images.
I'm still doing a lot of trouble shooting myself, particularly with my macro lense. I'm down right sickened by the results of my macro, starting to wonder if I need a new lense.
EDIT: Oh yea, also avoid a flash as much as possible. And if you have to use one, try to angle it at an opposing angle to the glass from the way the camera is shooting. Like if your camera is shooting strait into the tank, take off the flash and have it go off off to the side of the camera, instead of right down the view of the camera. That ought to cut down on any glare from the tank glass. (When I use a flash I tend the move it to the far side of the tank and use it as a side light)
I'm not too keen on using a flash in the tank anyways, scares and stressing things out.
