Strategy to charge tenants for water when not metered separately?

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I just purchased my first property. It is a multi-family with three units. I will be living in one, and the other units will have the same tenants who lived there under the old owner. They are paying below-market rent (one has a lease expiring next spring, and the other is month-to-month and the rent will be increasing gradually per advice from the property management company I am using). The water for the building is not metered separately, so I will be responsible for that bill, and the water district confirmed its going to be a pretty substantial amount (considering how low rent is). One of the units is a family of 6 who also has a swimming pool, and I’m wondering if there’s a way to charge a water fee of some sort without being able to show exactly what they are using. I want to be reasonable here, but I’m estimating at least 3/4 of the use is theirs. The other occupants would be me (I travel weekly for work... home 3 days per week max), and a woman with an infant and a toddler (her lease states water is included anyway). Is this worth considering, and if so, how can I be as fair as possible? Thank you for any advice.