The tenant did not pay the water/trash bills which were rolled into landloard's tax bills. How can the landlord get them back?

1. The tenants still have some unpaid rent. (9k) 2. The tenants did not pay water/trash bills which were rolled into landlord's tax bills. The lease stated that the tenant was responsible for all utilities(9k) 3. The landlord did not find that out until the sale of the rental property after the tenant moved out. 4. The tenant would like to file a small claim court complaint to get both back.