Help with ZEOvit reactor please

[surgeonfish]

How do you keep the water level in your ZEOvit reactors above the ZEOvit?

I have been using the ZEOvit system now for 2 weeks. I bought the GroTech ZF110 filter/reactor. I am turning the feed pump on and off at 3 hour intervals. I somehow thought that when the pump was off the filter remained full of water and the ZEOvit would stay submerged. Silly me. I checked the filter tonight and found that when the feed pump is off, the reactor slowly drains down to the water level in my sump leaving >50% of the ZEOvit dry. How do you keep the water level above the ZEOvit. I don't want to raise the water level in the sump if possible.
Thanks