How to address fish tank in rental?

[Maxim Lorenz]

I have a tenant in Texas who to this day has been great, always pays rent on time, receptive of rent raises, and overall transparent and takes care of the property. Our lease is set to expire 30 Sep 2022 and we renewed starting 1 Oct 22 with 3% rent raise. Today the tenant wrote me apologizing that he made an honest mistake of getting a fish tank a few months ago, which is prohibited per our lease and asks how I would like to handle it and whether I want a pet deposit. I am thinking of amending our upcoming lease with a clause of allowing the tenant to keep the fish and to ask for additional deposit. My concern of course if not the fish, but the tank (not sure of the size) and what happens if it bursts/leaks. What do you guys think? I do asks for pet deposit (tenant does not have any pets and now just the fish) and I do require renter's insurance per lease. Should I amend the lease asking for additional deposit and if so how much? I understand that renter's insurance will not cover damages to dwelling hence my concerns. Thanks.