Advice regarding tenant

[Estela H.]

Good Evening fellow BP's!

This is my first time posting in a forum and I'm looking forward to your responses.

I have great tenants in a duplex, two siblings in A &B.  However, this summer I noticed the water bill increased by 50%.  This was rather unusual so I reached out to them and asked what had changed.  The informed me that they had purchased a above-ground pool.  I wasn't sure how to handle it at first, so I didn't say much.  Clearly they took that as approval.  Here we are two months later and I have started a conversation with them regarding the water bill, liability and property safety due to the pool.  

They have offered to transfer the water bill to their name for the rest of their tenancy and are willing to sign a liability waiver.  This sound too good to be true!  Am I missing anything?  Should I add anything else to the lease to ensure that they run any major changes by me first?

Thanks in advance?

Estela