It's probably their way of saying anything larger than the standard "goldfish bowl" or "betta bowl". I can see their point though. If there's a leak it could cause damage to your apt. and the one below also. It may make you feel bad right now but you will get it back when you leave if there is no evidence that there was a tank there (ie. stained carpeting or ceiling below)
They are just trying to have money to repair things after a disaster if someone runs out on them. Another example of one or two bad tennants screwing up things for everyone after them.
If it is a NON-REFUNDABLE deposit I would tell them to shove it but I could understand them wanting enough money to make repairs if there was an incedent. Most places charge a dog or cat deposit and lots of them are non refundable.
Maybe this landlord has had an issue like this before and they just want to make it clear.... or maybe they are just jerks.
I'm not on anyone's side but I could see both parties' issues.
itZme