High water bill from running toilet, tenant or landlord responsible for bill?

Okay, I moved into a duplex last August and signed a year long lease. The lease agrees that we take care of the water bill which is split in half with our neighbors on the other side of the duplex. We've never had any problems, until now. My Landlord comes over with an $800 water bill (It's never been over $250) and starts checking our faucets and toilets. Turns out, the toilet in the back bathroom, which we never use because it's in bad shape, was slowly dripping. It could not be heard. He immediately turned off the water to that toilet and I explained to him that bathroom is never used and we had no way of knowing it was running. We have also had multiple plumbing issues since we moved in, which he sent his maintenance man out to fix. We've also had a leaky roof since we moved in which he is aware of and still hasn't gotten around to repairing. Our lease also ended a month ago and he hasn't yet asked us to resign, so technically we're not under contract with him as of right now and we are renting on a month to month basis. We have been excellent tenants. Never late on bills, we take care of our own stuff most of the time. IS MY LANDLORD RESPONSIBLE FOR THIS WATER BILL because of his old plumbing and cheap toilet, or are we? He is asking us to foot the bill. What should I do? I am contemplating telling him that if he doesn't like the fact we don't want to pay the overage, we will put in our 31 day notice since we are no longer under contract and he can take us to court to try to get us to pay it but chances are it doesn't seem it will end up going in his favor... correct? Please, any insight helps! Thanks in advance.