I would imagine it's an unapproved alteration to the plumbing fixtures. What does the lease say in regards to minor repairs or alternations?
I would send a letter to the tenant referencing the plumber's findings that upon recent inspection the plumber found that the aerators had been removed and the part in the lease (if it mentions it) that the tenant is not to alter or repair any items in the home without the LL's written permission. All repair requests must be submitted the LL...etc. If the bill still comes back high or they continue to abuse the water, raise the rent when the time comes at the lease renewal or with appropriate notice for your state if it is Month to Month.