After I asked my plumber to come to my properties and look for leaks, I came to the conclusion that my quadruplexes have huge water bills (water taxes?)

The Lease needs to be clear in its definition of what you are trying to accomplish here which is that the Tenant pays for all of the operating and maintenance expenses of the property.In addition be sure to include an understanding of who pays for major capital replacements (not just maintenance) such as replacement of the heating and air-conditioning units, water and sewer lines, electrical mains, roofs, glass, etc.

The hole in the floor in the basement was for the sump pump.

What is the age of the property and it's infrastructure (roof, water heaters etc?)

My advice is not to waste your money on testing clean with bleach water and buy a mildew mold suppressant from hardware store it's a chemical you put in a pump up sprayer keeps mold from coming back if on a painted surface get a fungicide paint additive and repaint area.

The property itself pays off itself after mortgage, taxes, insurance, sewer, and water - and leaves some to reinvest.

It sustained water damage from a broken pipe in the kitchen last winter.

I think central heat/air could go out of style if ductless heat pumps become cheaper and easier to install or if window/portable heat pumps become cheaper, more compact/sleek, or more efficient.

I only consider fire and water flood an emergency(a snake once too), most items can wait until I call the owner and get an approval anytime there is a repair.

Upgrade to water saving fixtures?