Instead of turning off the GFO reactor totally at times, you may try turning down the flow so that you can only see the top layer moving ever so slightly, so they process less water and last longer.
I'll try that too. In fact I'd rather do something like that, or limit the GFO in the reactor, because turning off the reactor intermittently will mess with the water flow in the system. Which would then trigger the ATO incorrectly, which would alter salinity, etc.. I'd rather have the water flow remain as consistent as possible. Same reason I hate turning off the skimmer unless absolutely necessary.