It is all about the lease ... You said you had new owners and manager ... they're trying to implement new rules.
If you're already operating on an existing lease that hasn't expired ... everything is mute. The lease should specifically state aquarium/fish tank limitations if any. It doesn't fall under PETS, just like waterbeds are specifically named in leases. Should it not say anything about fish tanks at all, then the issue is mute ... tank is allowed by law (check with your lawyer on that one to be sure ... different laws different places)
If this is a lease renewal, and now they want to implement new rules to the lease. They have to allow x number of days for you to relocate or readjust your living situation. I've dealt with landlord/manager/owner changes before ... it sucks. Use your lawyer as a sounding board, you do have a leg to stand on in all this.
But in my honest opinion, I'd start looking for a new place for when said lease is done ... because in the end they'll just screw you on the rent and jack it up.