Would fish count as pets in most leasing agreements?

This is just out of curiosity, but if you have a lease that clearly says "no pets" and you buy a fish, would that be violating the lease in all, most, some, few, or no instances?

Thank you, I want to know if in the future I ever rent a home and I decide on getting a fish.

That's something you'd need to discuss with each specific landlord. Mine don't count fish as pets, but since aquariums can cause fires (they really can), she doesn't like her tenants to have them.

Never heard of it myself but like all things it's best to ask the landlord before doing anything.

You should ask. It probably depends on the size of the tank---I don't think most landlords would object to a 1-5 gallon tank for a Betta. But the larger tanks can cause a lot of damage if they leak. My brother's apartment had it in the lease that they couldn't have a tank larger than 10 gallons. Since people aren't generally allergic to fish and they don't cause damage, I think your landlord will probably be OK with a small tank.

Everybody has beaten me to it.  I don't think anybody would count them as pets but most apartments aren't going to want you have a large tank in your unit for a lot of reasons, namely the damage they can in theory cause.

Like this:

I apts I live at allows 10 gal or under.

If it was a fish bowl or something, I couldn't imagine it mattering.  If it was me, I would do it anyway, but that isn't always the best route!  I have a small tank with a beta fish in it (3 gallon) and I have never told any of the apartments I have lived in about it.  If you wanted a large tank I would definitely ask first.  

Just remember.. the larger the tank, the harder it is to move it if you plan on moving!