Tenant Running Up Water Bill

[Shain Ismailovski]

I have a tenant that is currently running up their water bill to an absurd degree (Hot water is something I am responsible for). My last water bill was $1300 (It's never been more than $300 before bringing these tenants in.) It is a two family home, but I have never had an issue with my other tenants this only started happening when these new tenants moved in. I thought I had a leak but have had the whole system looked at and nothing was found. These tenants deny they are responsible for the increase. What course of action should I take as a $1300 water is not sustainable?