Geo all the Way!! Never had a single issue with mine when the 90 was running. Very simple to set up. Even Pete could figure it out! I think he might even still have one that he never set up.
Reactor (Nothing but Geo) LOL Ph controller Tank Co2 Regulator W/Solenoid. Media
Talk to Pete if you can get a hold of him. I think he has most of this that he planned on using but then never did for some reason. I know I h=got him a tank and he had a Geo.
I still like the Pro Cal by MTC (Marine Technical Concepts). They also make a Mini Cal. I prefer the two chamber Reactors over the reverse flow or conventional one chamber reactors. They better absorb the excess CO2 and also raise the alkalinity and ph of the effluent water better. Maybe ask Todd from Cherry Corals his opinion because I know he has used both a MTC and Geo Reactor in the past.
I couldn't imagine a SPS dominated system without one, dosing is a PITA in my opinion and thats why I switched. My reatcor is a ebay special (Acrylic City) and I would not recommend it but I would go Geo if I were to get a new one. His directions on setting up a reactor are very easy to follow too. http://www.geosreef.com/CR612.html
I had great success using a Milwaukee SMS-122 pH controller but 9 months ago decided to reduce the amount of clutter under my stand so bought a Reefkeeper Lite L3 and now run the reactor off its pH probe.