He lives in a fish tank? Offhand, I'd say that it's rather inadequate housing for guinea pigs. Cavy Cages has a page on why fish tanks aren't entirely suitable for guinea pigs. Most of it has to do with ventilation and space.

cavycages.com/cagetypes.htm

I've seen a few links on various sites that mention being able to catproof a cage, and several people have cats and pigs. Jessica over on CavyMadness uses a catproof cage.

cavymadness.ne.client2.at...proof.html
cavymadness.ne.client2.at...tproof.pdf

He could probably use a same-sex friend too. Guinea pigs seem to do better in pairs since they're social animals and if you're away for long periods of the day, it gives him someone to play with. My two boars have the run of a 2x4 C&C cage which basically gives them 10.5 square feet of space.

Now, if he's nipping at you when you try to pick him up, maybe you should use a small cardboard box and let him climb into the box before you lift him out of the cage.