Looks like a lighting problem??
So do you have the tank lights on? The room lights on? Your flash on? Are your room lights bright or dim?
Try this (if you haven't): have all lights on and take pictures physically up-close to the tank at a slight angle -- so the flash isn't reflected on the glass or if it is, it's in the corner. Try shooting at different angles.
If this isn't it... how old is the camera? Maybe it has a white balance problem. If that's it, try taping a white piece of paper at the bottom of the tank and take a picture of the tank with the paper at the bottom. You can always crop out the paper later.
Tell us what happens; I'm very interested.