Utilities owed by active tenants

[Glenn Kendrick]

I bought a duplex in October 2018. The water/sewer bill must stay in my name and be paid by me but under contract the reimbursement of the usage cost is the responsibility of the tenant. It took two billing cycles before I saw their bill so the owed amount for their unit is $425. They always pay but are usually late with rent. Almost one month ago I sent them their owed bill with a letter that said to pay in full by 5/30/19. What can be done if they do not pay?