I think the pictures look better with out the flash. But if you need to use the flash, try taking pics at a angle so the light reflects off to the side and not straight back. I'm just getting into this Camera stuff, so Im just a camera noob myself so take it for what it is.
I just got a camera from cc that i tried today. Its the new olympus 1030 sw and it amazing. I have small catfish that never stop moving and it captured then great with the multi shot settling and it has a thru glass setting that works great.
Now for the best picture, this digital camera is WATERPROOF! I stuck it in the tank and got great pictures!
If your willing to spend around 350 dollars then you can get rid of all the tank picture problems.
personal choice... i personaly hate olympus and fuji cameras and swear by sony
for simple point and shoot digital cameras ive never found anything better than the cybershot series.. the one i have now i got used from someone who had no idea how to use it and he swore it was a total POS and couldnot be made to take a clear picture ($250 camera i got it for $60 with everything)...
out of the few thousand pics ive taken with it if only a few have come out blurry... but i know what this camera can and can not do..
Any outside light sources around the tank will cause glare. I recently took pics of my community tank, and ended up closing the blinds and making the room dark so that the pictures would turn up clear.
I've been tempted to put a sheet or something over the tank and taking pics inside it, but haven't felt that motivated.
Tripod, high exposure (if fish are very active and you have enough light), no light sources behind you, etc.
Like most have said, it's the photographer more than the camera.
For good full tank shots, get lots of light inside the tank from above, and try to avoid using a flash. Also, try to remove any lightsource hitting the tank from the line you are seeing (lights from behind you).
For taking pics of moving fish up closer, work on panning with your hand to keep the fish in one spot in the frame. It will blur the background while keeping the fish clear and sharp. You'll end up with some amazing shots.