Hold your button on your camera half way down so it can focus and then shoot. Another way is to turn off your lights and leave your aquarium light on. Make sure your camera is not set on micro and your trying to take pictures from a distance or your trying to take close up pics and your camera is not set on micro. Try messing with your settings on your camera take pics and see how they come out.
My good fish pics all came with patience......
Yeah, the lights off with the fish light on worked fine!!
Check out this pic which won on another sites potm......
Even if you don't have a remote flash or fancy camera you can still learn a lot from that thread. They cover a lot of stuff throughout as far as lighting, angle, settings and so on. Good luck!