Toilet running for months. How do I recoup the cost?

[Mitchell Pudelski]

I have a duplex with long-term tenants who pay on-time every month. Problem: water bill has quadrupled for each of the last four months. Nobody has an answer. Each reports no leaky faucets or toilets. Upstairs tenant states the toilet runs every now and then but not much. I adjusted the tank level as it was close to cresting the overflow tube. Maybe that was the issue. She stated she even replaced the flapper herself. The chain was too long though, so I shortened it. Every time I visit there are no problems, until yesterday. I showed up unannounced yesterday and found the chain wrapped UNDER the flapper so it was a continuous flush. Tenant was sleeping and did not even get up with me loudly announcing my entry. Remember, I had previously shortened the chain position but left the additional chain attached. It required the user to hold the handle down for a full flush.. It was lengthened again when I found it. To put this into perspective, this was a $420/month increase x 4 months! How do I get almost $1700 back from her?