Thanks for all the comments. To answer some of the camera questions above:
I use a Canon 350D and Canon EFS 60mm macro with Hoya +10 and Jessops +4 closeup filters attached to the front of the 60mm lens to reach a +40 magnification. The camera is 8MP so one can crop the image quite substantially to 'digitally' zoom in without loosing resolution. Most pics (>95%) were taken without flash relaying on tank illumination alone. All pics taken handheld, no tripod was used as creatures moves so fast that using a tripod becomes impossible!
Ideally, I would like to have an external flash gun which can be triggered remotely so that it fires above the tank (Sun light always comes from above!) to create a more natural look compared to light comming from the on-board flash light. As I do not have an external flash, I seldom use the on-board flash because it would create an unnatural look.
Without flash, one has to balance between Aperture and Shutter speed to allow more handholdability, ie a faster shutter speed without sacrificing depth of field (DOF). It is difficult, as my priority is to increase DOF (with smaller aperture) so that the whole animal could be in focus, but this will decrease shutter speed (longer exposure time). This explains some of the blurry shots in my gallery because of handshake in the attempt to increase DOF.
All you need is a great amont of time and patience in order to capture the creature when they momentarily stand still! Or otherwise spend some more money to get an external flash!