If you have an agreement where the water or other utilities are included in the rent, then there isn’t a way you can change it at this point. Whether the water is provided to you at a flat rate or on a metered basis, you cannot reduce or remove the service without their permission. If the tenants do agree to take on the responsibility of paying their own water, you would have to reduce the rent to compensate (using the average monthly cost of the reduced service) for the removal of services that was previously included in the rent.