What I find useful for taking close-up shots is the "Macro" mode on the camera. This allows you to get nice and close (around 10cm away) and get good detail in the shot. The super important thing with this mode though is to keep the camera very very still. It's even better if you have something to sit the camera on or even a tripod.
The other important thing is lighting. The tank should be well lit and obviously taking a shot through glass you can't have the flash on.
Your camera has a Macro mode on it - the button / selection looks like a little flower
Hope this helps!