Just about any digital camera can do what you want.
All have Macro mode, that will let you take photos inches away from the fish.
You should not use flash unless you have the camera lens pressed against the aquarium glass, because you will get a glare off the glass.
A good choice in the $150 to $250 range is the Panasonic ZS15
If you want to start with something less expensive here is what Canon has to offer: