I recently asked my dad whether his condo has an aquarium restriction, and apparently it does, too. They let barking dogs in and one of those dogs knocked my mother off her walker, which is a lot more dangerous than, say, having a tank leak.
I can think of 3 worries that a condo association would have: 1) the weight of a large tank couldn't be supported by the kind of construction in the building, particularly a wood-frame building 2) a big tank could leaking and damaging another person's apartment, and 3) the condo association has property insurance which would be voided if there is an aquarium in the building.
If your apartment is on the main floor (concrete underneath) or it's a concrete building or you have had the condo inspected, then the weight of a tank is probably not an issue. In that case, perhaps you could ask the condo association to allow you to have the tank? That way, you could be upfront about it.
Otherwise, I agree with Scholz that you could just move in with the tank and not say anything, especially if the tank wasn't sitting in plain sight of the door.
However, if the neighbours in the building are at all as friendly as you'd like them to be, chances are your tank will be discovered one day, and by a member of the council. Some council members, particularly seniors, can be sticklers for rules and you could end up in in an unpleasant situation, not only arguing with the council and feeling bullied by them, but also casting black glances at your neighbours. That would not make for a very happy home.
I'd probably ask the council if it was possible to have an exception for your aquarium, particularly if you have a main floor suite over a concrete parkade. But, that's just me. I really dislike conflict with my neighbours.
