Looks like the shutter speed was still too low. When you have the camera set to one of the scene modes you can't adjust the shutter speed- but you can put the camera in manual exposure mode and set both the shutter speed and aperture yourself.
Give that a try- setting aperture to f2.8 first, then experiment with the shutterspeed to see how fast you can set it before the picture becomes underexposed [dark]. You may need to set the iso up to 400 to get enough speed. Hopefully the shutter speeds will be at least 1/125-- that should be enough for crisp pics.
Couple other tips- try to shoot squarely into the tank so as to minimize distortion from the acryllic/glass. Another tip is to get the autofocus locked on first- by holding the shutter release button halfway, wait til it locks on, then wait for the fish to be in the right place [same distance as the point you focused on] and then press the button the rest of the way down to take the pic. Most point and shoot cameras have a big lag time between when you press the button and the picture is actually taken- getting the focus locked minimzes the lagtime.
