Most fish will frequent certain areas due to the whole territorial thing, so setup your camera on a tripod and pick a specific area for each individual fish. Then if you have burst mode or multiply frame setting pre-focus and lock your focus until your fish swims into that specific area and simply release the shutter to shoot multiple frames which might or might not result in a great image, for every 40-50 fish images i get one or two keepers-thank goodness for digital cameras!!!
Adding some food before hand is also a trick i use, it slows them down a little not to mention it keeps them smiling
