You guys know a lot about fishkeeping, but you don't know beans about photography. On the flip side, I know a lot about photography and I don't know beans about fishkeeping.
slowlyburn, I'll bet that your problem is not your focusing technique, or your aim. Most digital point and shoot cameras lack the light sensitivity (400 ISO is usually the max for point and shoot cameras) to be able to use a fast enough shutter speed to freeze an animal's movement in a relatively low light fish tank. That is to say, when the shutter opens, it allows light to strike the camera's sensor, making a picture. When there is not a lot of light, typical of most indoor settings, the shutter has to stay open longer to allow enough light to hit the sensor for a proper exposure. When the shutter is open longer, animals and people have more time to move, leaving the subject blurry as if it were steaking across the image. Unless you use your camera's flash, you will most likely be left with a picture of a blurry fish.
Another possibility is that your are closer to the fish than your cameras minimum focus distance. If your camera has a macro setting, try using it together with the flash and see if it helps.
When I take photos of my fish, I use a digital SLR (Canon 20D) with a 50mm f/1.4 lens. This is a lens with a huge aperature that allows the maximum amount of light to hit the sensor. I also typically shoot with an ISO of 1600. All this allows me to use a fast enough shutter speed to freeze a moving fish.
It doesn't matter how new your camera might be. You may just be asking it to do something beyond what it was designed to do.
Here are some pics of my fish:
Yellow Acara
Motaguense
Beani Hyrbid