You shouldnt use an aquarium ever, no matter how large. Guinea pigs like ground space to run not levels really. It wouldn't hurt to make it a third level but it might just complicated cleaning and catching piggies for you.
I'd say if you have the space to expand the C&C cage out rather than up do it.
Aquariums are harmful to a guinea pig's health for they do not provide proper ventalation and should never be used. Like Kim said, guinea pigs don't really like a lot of levels. Just extend the base if you want to give your guinea pigs more room.