Ok,
1st
Read up on shooting in Manual Mode.
You want to shoot with pumps off
Learn about aperture vs shutter speed
If the camera can do custom white balance (to get rid of blue hue).
Use a Tripod for long shutter speeds (low light)
Try to take pics with camera perpendicular to the glass. Any angle may distort the picture
Shooting top down will give AWESOME SHOTS!
I never shoot with a flash for my tank.
It is hard to give tips, need to really just play with the camera.
If the camera has a custom white balance feature, you take a photo of something WHITE in the tank but you want it to take up as much of the photo as possible. Then you use that image to set the white balance. The camera will take that photo and adjust it so that it knows what what is.
Check your manual for custom white balance. And depending what kind of lights you have you can also use a "K" setting, just like the bulbs. But I have found that the camera will not go high enough with the K setting.