You would need a by-law changing the basis on which water usage was calculated (and I’m not 100 percent sure that would be legal) but even if you got the 75 percent of owners to agree, how would you be able to check how many people were actually living in a unit without their cooperation? In short, I don’t think the head (or bed) count method will work.
A more elegant solution may be to pass a by-law allowing owners to install approved individual water meters at their own expense, but for their water usage to then be extracted from the general bill, and based on their general usage.
Written into any such agreement would be clauses about the integrity of the meter and its availability for inspection.
Gradually, then, the over-users would be isolated and would be paying an amount that truly affected their water usage. Retro-fitting water meters is neither cheap nor easy but it does make the system a lot more fair and is good for the environment too.
An ‘opt-in’ system would get round issues with meter reading and enforcement.
Have a look here to give yourself an idea of what’s available.