Had this same problem with my Reef Octo elite 150. Wouldn’t stop overflowing no matter what. Final solution was to run the cup overflow into a 5 gallon bucket, cranked it up and let it do it’s thing. I have a 140g so after about 10 gallons of “overflow” it went back to normal. No idea what caused it it that was the solution. Pain the the rear for sure but it worked.
Letting it overflow back into your tank is just keeping whatever is in the water, in the water.
GL