Just curious, did you buy the fish tank at the pet store? Of course they would say that it's a good cage. They're making money that way. I don't think that a glass tank is the same as the kind of aquarium things that the pet stores keep guiea pigs in prior to purchase. Those are made of plastic, and there's a definite difference between plastic and glass. I've kept hamsters in glass tanks because they're nasty-minded little escape artists, but it's tricky to make sure that the ammonia buildup doesn't become a problem. Since you have a cat, I'd wonder if you have some sort of mesh cover on the top of the tank. That probably cuts down on the proper amount of ventilation that can get through as opposed to it being an open top. Not to mention the amount of heat that might build up in the tank. It's made of glass, and you know how a car dashboard gets when the sun shines through the windshield, right? Add in ammonia buildup from urine, and it could turn into something like a sauna pretty fast. Glass tanks just don't provide enough ventilation at all.

I would also hazard a guess that the reason why your Piglet might be prone to nipping and such is because the glass walls block off his ability to smell his surroundings, so he feels less social. Think of it as living in an area the size of your average king-sized bed with a clothespin over your nose. You can't smell a whole lot, and for an animal that relies on their sense of smell in addition to sight to react to their surroundings, it's almost like a disability.

To be entirely honest, I wouldn't keep a guinea pig in an aquarium, they're just too inadequate for the needs of the animal. Besides, most pet stores have somewhat antiquated views about animal housing. Most websites I've seen about guinea pig housing don't reccomend aquariums as housing, and I've seen very few books that do either. In fact, PetsMart.com says that aquariums are not appropriate.

Did you check out that .pdf link that I posted? The spacing between the cage is very small and I doubt a cat could get their paws into there. You need to be fair to your guinea pig by providing him with a more ideal habitat than a glass tank. He's not a reptile or a fish, and it's not fair to him to live in a glass tank. It's up to you to provide a good home for him and that includes a safe, well-ventilated habitat for him to live in. It may take some effort and work on your part, but you should be capable of providing a cat-proof cage that is not an aquarium. It's not fair to Piglet to have to play second fiddle to a cat and be inconvienced just because you have a cat. There are lots of people who have cats and guinea pigs, and they find ways to provide a good habitat for both without shortchanging one or the other.

Also, does your cat have free range throughout most of the house? Or at least several rooms? If so, it's doubly unfair to your guinea pig because he's stuck in a glass tank that can't possibly be more than a foot or so wide by little over two feet long while the cat gets the run of the house. I suppose if you absolutely had to have an aquarium for your pig, you'd need one that was acrylic or plastic so you could drill ventilation holes into the thing. But those kind of aquariums are terribly expensive (at least $170+) and you could probably make a catproof cage with organizational closet shelving for far less.