to get best possible pics without flash bump up the ISO as much as you can - it will give you the ability to use faster shutter speed (cost is a tiny bit of 'noise' grain in the photo)
also far better to zoom in as little as possible. Get closer to the tank rather than using any zoom (that is if it doesn't make all your fish hide). Less zoom means more light onto the camera's sensor, and therefore again ability to use faster shutter speed. Also most small cameras have variable aperture lenses, and the aperture is wider when the lens isn't zoomed than when it is.
PS congrats on the pics you've taken which are lovely. Nice tank too! Love the rock-work