I have a Sony DSC-F828 which is now 6 years old. I aim, wait for the "beep beep" that tells me it has focussed on something, then I push the button. I do this hundreds of times and sometimes I get 1 or 2 good photos out of it 8)
To photograph the fish and tanks, do it at night with all the curtains pulled to stop reflections. Have as much lighting on the tank as possible (and I turn off the main room lights) and the shutter speed as fast as possible (so I have been told but I don't know how to change it). I have tried reading the instruction manual but it appears to be in Greek and it uses lots of terms I do not understand.
It helps to have the camera on a tripod, or box, to keep it steady. If you use the flash, aim at a slight angle to avoid the flash reflecting off the glass. I have a separate flash I can attach (more powerful than the one built in) which helps.
However you do it, prepare to get lots of useless photos and only a few goodies :roll: