2 moves ago, the place I was at considered anything over a 10 gallon tank a "pet" = pay pet deposit. On each pet. I don't know if they had a maximum size, my tank's 38gal and they were fine with it. They didn't like you having more than three official pets in the apartment though, maximum two cats, no dogs. Caged animals also counted as pets. This was a problem when they found out that me and my roommates had between us two cats, two chinchillas, three rats, two mice, three corn snakes, a leopard gecko, two 10 gallon tanks, two betta tanks, and my 38gal fishtank... The big tank and two of the tanks were on the lease. They gave us a week to move out, or get rid of the animals. We ended up rehoming or fostering most of the little guys, gave the small tanks to friends, one of the cats got sent to live with his owner's parents, and I got to keep my chins and fish.
My current apartment doesn't really have a maximum number of pets, as long as they know about any dogs and cats (prefer under two of those, and they don't care if it's in a cage/tank), but I went to ask about larger aquaria anyway, unsure of what the place could support... they ended up deciding that 55gal was the max they'd allow. Apparently the question's never come up before. I know that when I moved in, they wanted to know how big my tank was... maybe no-one else here has a larger fishtank?