both cameras are cpable of taking a good picture but thats usually what you get when you takae a close up picture of something moving without using flash. try to turn the lights in your tank off and hold the camera 4-6 inches from the tank use flash and heres an obvious one make sure your lens is crystal clear.give your camera a second of being somewhat still to auto focus,if you dont have a stand hold your hand steady with your other
nyc, your shutter speed is too slow for no flash. As pps said, use your flash at an angle and hopefully it won't create a hot spot on your pic. Either that or light your tank really well using the tank lights as well as additional surrounding lights and increase your shutter speed. Fish move really fast so shooting them without a flash and having them come out without blur is pretty difficult. HTH!
I forgot to add, use your macro setting! This is a short plane of focus but it allows to to shoot very close up minimizing the possibility of seeing the flash in the shot. Here's a couple of pics I took w/flash set on macro...