Hey Tony, I have the same Red Sea co2 reactor and it does a very good job. But you have to keep it clean which means washinging it out every 3 weeks. Since you haven't done a water change for a while, i'm sure it's efficiency has been minimized.

Actually, a lot of plant people are moving over to sumps. You do have to crank up the co2 a bit but it should still be measureable (unlike your circumstance). Tom Barr (estimative index - plant guru) has a thread on his site going about his own tank with sump. It's been doing very well and he has no problems optimizing the co2 levels.

I would keep that Tunze turned on, just don't point it towards the water surface. You just want enough surface agitation to move a few circles around. With your overflow, there is no need to worry about any scum build up.

How is your drip plate setup? Is the water tumbling into a filter pad and then dispersed via a drip plate?

I read up on the phosban reactor and it is too small to run GFO on a 120 gallon system. Unless you want to trade both of them.