@Justin K.
It is possible to put a meter behind pretty much anything some places have them behind sinks and toilets to monitor usage. Manual readings would obviously be much cheaper to install and you can check them every quarter.
A better suggestion is to replace the aerators on all 6 units. From the bathrooms to kitchens. Every bathroom sink install a 1-1.5gpm aerator and for kitchen sinks I wouldn't go below 1.5 gpm. These are only $2 a piece and take 30 seconds to install yourself. They are typically chrome colored and can be installed on any faucet. If you have a fancier kitchen faucet it may not work unless you get one for the specific model. Note: most older sink aerators use 2.2 gpm
Also replace shower heads ($6 a piece) also a 30 second DIY with 1.5-2.0 gpm shower heads. You may get some complaints at first until tenants get used to them or decide to go with 2.0 gpm on all units. Note: most older shower heads use 2.5 gpm
This will drastically reduce water consumption.
The last and most expensive option is to replace toilets to low flow models. They are typically around $100 and take time to replace so you may have to pay labor on that as well if you can't do it yourself.
Note: older toilets can use up to 4-5 gallons PER flush. Newer models are about 1.5 gallons per flush - if you need a recommendation on a low flow toilet that won't clog just PM me.
I would start with the aerators and shower heads since they are very cheap and can be done in an hour on all 6 units. Make sure to market this as an "improvement" to your tenants.