I absolutely loved this article on Aquarium Photography by Ashok Kandmalla: http://www.appshyderabad.org/basicsofphotography.php?id=70
Indeed helped me to take some nice pics of fishes..
Few important points from the article:
# The ambient light in the room should be kept at an absolute minimum to avoid reflections off the front glass of the aquarium
# If accessible, cover the backside of aquarium with black non-reflecting paper/cloth
# Clean and filter the water so there are no floating particles in the water, as the flash may illuminate these.
# Turn off the flash in your camera if possible.
# Hold a high wattage table lamp at the top of the aquarium (above water) pointing it downwards with the head parallel to the surface of the water.
# Put a suitable diffuser like tracing paper (or thick plastic sheet) to soften the light. Do not touch the bulb!
# If your camera allows then set the camera exposure mode set to manual exposure and the shutter speed to the fastest sync speed that the camera supports to avoid ghosting. You can keep the aperture somewhere around f11. If the function is not provided, select the mode which allows you to take outdoor photo (Do not select Night mode)
# The front element of the lens should be parallel to the aquarium front glass (that is lens axis should be perpendicular to the plane of glass)
# Mount the camera on a tripod and pre-focus at a plane in the aquarium at which the subject is likely to appear (about at an inch or two behind the front glass). ***If you do not have a tripod, place it on a table and few books to adjust the level. Keep the cam over a towel so that it doesn't slip
Hope this helps!