Why don't you just use a CO2 reactor on the outflow? That is what I do.Hah. That's what I was on here searching for: how to find fittings that will work??? I'm trying to run my CO2 into the filter return thingie. Seems like Fluval uses odd size tubing. Makes me crazy, cuz I wanted to hook it up right now!
The secret to using your Fluval is to know that standard 5/8" Id pond hose connects to the filter just as well as the corrugated stuff it comes with.