Yes, definitely do NOT keep them in an aquarium! That's barely enough room to move around once they're grown and they will be exposed to massive amounts of ammonia from their own urine.
I would recommend reading guinealynx.info to get more information on guinea pigs in addition to the links provided by canadiancritter; it sounds like maybe you jumped into piggie ownership without a ton of research!
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry