Will adding a snorkel tube with one-way plastic check valve help?
Outflow of filter is only 1 inch above water level in aquarium.
Filter is open on top and there is no pressure in the system.
Anything that significantly restricts water flow in reactor will
cause it or filter to overflow.
Long tubes, check valves etc will create too much restriction.
This is the biggest problem of this system. You need to adjust water hole
in reactor to make it big enough to let water flow trough without causing overflow, but small enough to prevent CO2 from burping though it.