Verbal agreements between you and the landlord mean nothing in a court of law unless there are witnesses to the discussion.
What does the lease say?
If it says you can have one aquarium up to 30 gallons, then you are in breach of the lease. If it says you can have aquariums up to 30 gallons, then you are ok.
If there is nothing in the lease about pets/ aquariums (unlikely but possible), and there is only a verbal agreement, then you might be ok. If the landlord says aquariums up to 30 gallons, then go with that. If they said 1 aquarium up to 30 gallons, say you thought they said aquarium/s (plural). But be prepared to shut a couple of tanks down if they tell you to.
And I assume you have the pet insurance so they are covered if something goes wrong with the tanks?
They have no ground to evict you if the agreement (in paper with a signature) say nothing about hoe many tank you can have. If they choose to take you to court you will win. You may be even able to counter sue if that comes up. But I doubt it will.