If the parameters are best camera for under $250, maybe we can offer some suggestions, but it won't necessarily be the best camera for taking pictures of your aquarium. I think with a decent knowledge of photography and cameras you could get decent tank photos with a Fuji S700, Canon A630, or Canon A720. Those I believe all come in under $250. Just do some research and see what you want out of a camera and are willing to pay for.
FWIW, the above answers weren't all that sarcastic given the criteria you supplied.
Dave