TLDR: I'm going to reinstall the float valve that came with the ATO. Fault was mine for adjusting ATO system without realizing water level was now higher than the sump.
Thanks for the suggestion Randy.
I'm doing something similar. My refugium sits on top of the sump (a converted, drilled, RSR ATO tank) and it is plumbed using 1/2" hose; the return from the refugium is gravity fed back into the sump's return pump chamber.
Into fuge's return I have a 1/4" line that is connected to a pump that sits in the ATO vat. (Neptune PMUP.)
Here's the problem: my vat's water level (when completely filled) is higher than my refugium return line. I fill the vat only once a month or so. I made some adjustments yesterday (I raised the water level of the vat) and I forgot what problem this could create if the water level got higher than what it has been set to. Doh!
Salinity dropped a bit -- probably 2 gallons of RODI into a 65 gal system. Glad I caught it -- score for Neptune Apex's altering me; even though the salinity reading my Neptune probe is incorrect (like most), it's consistent.
I'm going to reinstall the float valve that came with the ATO. Fault was mine for adjusting ATO system without realizing water level was higher than the sump.
Thanks again.