Water sub-metering - Is there a way for me to sub-meter my older, brick 12-unit?

[Karl Kauper]

I own a 12-unit, 3-story brick MF in Euclid, OH. I am paying the water bills because the units are not individually metered for water. I believe that to install conventional water meters would be cost-prohibitive, but I just decided to send this post in case anyone has any creative solutions that would help me save on water bills? I don't mean low-flow solutions (which I've already implemented) or raising rents. Rather, is there, for example, a way to track tenant water consumption without replacing the entire plumbing system throughout the building so that meters can be installed? Thank you for any insights/suggestions! - Karl