I recently went through the painful task of deciding on a calcium reactor for my two 90 gal tanks. (Soon to be barebottom, SPS tanks!)
My final decision was for either a GEO or a Korallin.
The Geo has a great track record and gets high marks for reliability and excellent customer support. (Basically the company will do anything to make its customers happy)
The Korallin has a similar track record, but nothing is really mentioned about customer support. (Never seems to be an issue)
My decision came down to what one should be easier to use.
The design of the GEO recycles CO2 that makes it all the way to the top of the unit. This must be monitored or else the pH will drop too low and will turn your media into mush. In order to avoid this risk, a ph controller unit should be used. A pH probe is a minimum requirement for this model of calcium reactor.
The Korallin models have the option to either recycle the CO2 or to allow it to accumulate. Its just a slight design difference, but it is very important. You can run a ph controller and recycle the CO2 or you can skip the extra equipment and run the unit in a manner that allows the CO2 to accumulate at the top of the reactor.
If you do the latter, you can manualy adjust the amount of CO2 going into your reactor (bubble rate) untill you no longer are getting the accumulation of excess gass in the system.
Careful though! Too much CO2 accumulating at the top of the reactor and the pump driving the unit will suck air and lose siphon.
The nice thing about the Korallin is that the pH can only get to a certain point before it stops lowering (when used in the non-recycling manner). This prevents the media from turning to mush on you.
I went with the easiest unit to set-up and to run. Korallin.
Look up Hahnmeister. He is the person who provided me with the information I used to make my own decision.