I would also take this opportunity to remind them what does and does not get flushed down a toilet. 

For example, on one of our properties, we installed all brand new low flush toilets and reduced our water bill by 55%, a massive swing in usage. 

Well, people who are flush with cash or equity from the last 2+ years of crazy appreciation in real estate and the stock market. 4) Some investors are OK with buying low to negative cashflow for the first 12-18 months while they stabilize the property, increase rents, and fix deferred maintenance items.

Or after we went in and flushed the toilets in the vacant unit (per our contractor it's because the P-trap was dry from not being used in the vacant space). 

New circuit for the garage door opener, new LED lighting and circuits for the garage, Flush mounted LED lights (disable) for the the kitchen, New led lights for walkways, hallways, bathrooms etc.

This is the simplest method, but it also means your tenants are more likely to abuse the utilities by leaving windows open with the heat or A/C running, leaving lights on, ignoring the toilet that constantly flushes on its own, etc.3.

Another time they sent in a service call saying the toilet stunk when they flushed it.

Every 2 years I do a 10 min descale/flush of each system.Now planning the next install in a large high end rental home we have.

Manchesters water is absolutely terrible for these units and if they aren’t flush on a regular basis you will have blocks and all sorts of issues that will lead to higher expenses down the road.

You can also charge her for the excess water use.If there's no evidence of extra occupants, the excess water may be due to a toilet that constantly runs or flushes.