Is it an external reactor? Is your reactor above the sump? I haven't used one, but I have used external zeovit reactors and generally the easiest thing to do is have the reactor or the outlet of the reactor below the sump level. This way when the reactor turns off it should break the siphon and stop the flow from draining the reactor. Another option that is a bit trickier and might work but might be unreliable is to create loop in the tubing an arc that is higher than the reactor, so the flow goes up and then down into the sump. That may help as well. I assume your running 3 hours on and 3 hours off in the early stages? I had a similar problem with my vibe, I just put a crate outside my sump for the first month so the vibe was lower than the sump, annoying but once I switched to 24/7 this is non issue anyway.
