[noy]

The manual/custom WB button is for tuning your WB and best done using white cards (not sure how practical that is in an aquarium setting). I suppose you can put a white plastic card in your tank and use that as a white reference.

Graininess is often because the ISO setting is too high and you get distortion from that. Check you photos to see what the ISO setting is. 800 is the highest I shoot at unless the subject is moving. If you use auto ISO - often the camera will push the ISO setting to one that is way too high. I shoot most of my shots using Aperture priority and use a tripod and all fans off so I can shoot with a slower shutter speed and still get sharp photos. If you can keep the ISO below 400 - those are the sharpest shots.

Can you post some photos so we can see what the problem is.

As for white balance - photos taken in actinic lighting will always be too blue. I'm not sure about the G9 but try to shoot in a WB setting that's as close to 10K as possible - you may only go up to 7K but that's a start. Too green - often that is from regular florescent lighting - that's easily fixable with the right WB setting. Sometime the blue in the actinic is just too strong and you will have to do some post shooting white balance adjustments in Photoshop (or some other software like that).