Well.. I've heard of professional breeders keeping a pair in as little as 1 square foot. Personally, I wouldn't even keep a single bird in less than 2x2 ft unless it was a very temporary thing. 2x4 ft seems to be a reasonable cage size for 1-4 birds (that get along well) to me, but more room is always better.
I really think they should be allowed to move about, there should be room for artificial plants for them to hide behind, room for a dust bath, room for a dark corner to nest in, room to get away from bullies and scary humans and room to stretch their wings.
If you really want to use the aquarium, I certainly wouldn't put more than a pair in there. There is a higher risk of fighting if you keep more than that and with so little room, they don't stand a chance of getting away from each other.
I kept 2 sisters that were raised together, with a roo in a 2x5 ft cage this winter and one sister actually started chasing the other when spring came. I then relocated them to a 3x10 ft aviary, hoping that would stop the chasing or at least give the one being chased a better chance. I haven't seen any chasing since, but haven't been watching much either. But don't expect to be able to keep trios or groups in small cages.
I have a group with a roo and 3 hens in a 4x7 ft-ish budgie aviary and they do so well they even hatch out their own chicks on a regular basis. They even help raising each others chicks. There are 6 chicks in there now and I think one of the hens is already broody again. The ones I keep in smaller cages(2x5 ft & 2½x2½ ft), haven't had any luck with the incubation yet, they've been broody a few times but left the nest within a week. I interpret this as stress caused but too small living quarters. But there are people who have buttons that get broody and hatch their own eggs in small cages as well, though it seems to be a less regular occurrence than in my aviary.
You might not care for having parent raised chicks, but I think it tells something about how happy the birds are with their living quarters and my conclusion is that whereas ~2x4 ft might be the best I can reasonably do for the birds I keep inside, 4x7 ft is better
Keep in mind that they tend to boink into the top of their cages if something scares them. This can cause issues (from loss of feathers to immediate death) if the cage height is between ~15 inches and 5 feet or something like that. To prevent it, I pad the top of my cages with foam rubber or suspend a layer of tulle or similar 1-2 inches below the cage top.
