Thanks for your reply. What if the tennant is on vacation and the water heater leaks. How are we going to find out other than seeing the water? Is there any devices you'd recommend for finding the leaking water heaters remotely before the tennants do?
Also, do these valves typically leak a little bit before they leak a lot?
http://www.plumbingsupply.com/floodstop-water-heaters-conditioners-filters.html- this page lists ones for $120. I can't afford this for every apartment. Are there any less expensive ones you know of? I think that the cut off valve just needs to connect to the shut off if water is detected.