Idk what conservice is but find out if your sub metered or allocated a portion of the overall complex fee (this could be stated in your rental agreement).

If your sub metered, I think you probably have a leak because that seems very high. If your allocated a portion of the overall water bill I don't think there's anything you can do about it until/unless you can convince your landlord to support that figure (which as far as I know they're not obligated to, that'd probably be a question for a lawyer) and hope that they've got something incorrect in their calculations.