Ew, I don't like check valves (in general) but I'm not sure you really have any other option in this case. If you did a tubing pipe loop that would work, but only if you had a siphon break at the top of the loop (otherwise it will still just back siphon), then you have the issue of a little water stream spraying out of it when the pump is on.
The only other thing I can think of is try a sort of mini sump. Ie. a 10g tank on its own that you can have the water level where-ever you like and have the reactor sit in there. You could direct water from your overflow into this tank and then overflow from there back into the main sump.
The drawback to this is obviously the practicality - unless you have a lot of space and/or a dedicated tank room it's probably not an ideal option either.
The simplest is probably the check valve. (Did I mention "ew" yet?
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