Extra Problems with Aquariums
Yes, they're too small, and very poor with ventilation.
Other problems I've noticed back when I used aquariums:
ESCAPEES! Guinea pigs learn, and lean quick, how to get out of these buggers. They can jump- HIGH! They will escape if they are lonely (and it's too cramped to put more than one in there- it's not even good for 1). You can't really put a lid on them either all the way because the water bottle gets in the way
CLEANING- this is a pain in the butt! The aquariums are heavier, and more easily broken! With a cage, you can remove the bars part and dump the bedding into a dumpster or trash bag. With an aquarium you have to scoop the waste out, clean the sides (all the way up) and give it time to air out so the fumes don't hurt your piggie.
NOWHERE TO HIDE- pigs need little hiding places in their cages, and places to burrow. If you put houses or something in the aquarium, it takes up valuable space (which there's none of anyway) and gives them a stair to escape on!
COST- aquariums are more expensive than cages, even at a pet store. I made a C+C cage, and I got my coroplast for free at a real estate office- they just gave me their old signs. The cage cost me $20 (for the cubes) and it's about 10 square feet, and 14 inches tall. Plenty of room for hiding places, hay, food, waterbottle, and bedding.