Cant figure this out for the life of me!

[Slick Fork]

Use a tripod and aperture priority mode to get the depth of field you want. Depending on lighting and your camera you can be between ISO 400-800 (1600 on mine) before you see any real "noise" in the image.

UV and polarizing filters aren't really useful in fishtanks, at least I haven't found them to be.

As for white balance, I shoot in RAW mode and then adjust on my computer when developing the shots.

Biggest tip though is just shoot and shoot and then shoot some more, it doesn't cost you anything but your time and practice is the only surefire way to get to know your camera!