Since this is a charge for utilities, it would not be considered rent and you cannot serve the arrears of rent notice. In this case, your recourse would be to serve an N5 notice to evict the tenant for interfering with your lawful right. As of September 1, 2021, landlords can file an application to recover utility costs from the tenant. The prescribed forms are available on the LTB website http://tribunalsontario.ca/ltb/forms/.