As far as brand goes the nice ones are the jet streams and the aqua care ones. You want to buy it with a pH controller that way you are not sitting there counting bubbles and making sure it is consistent. The controller will automatically shutoff the co2 when the desired ph is reached in the chamber. For the extra money the controller is worth it. Also make sure your probes are calibrated correctly every 6 months. (usually when you refill tehe media and service your reactor)
The media that you put in there is also important, some reactors use coarse media others use fine media. If you use fine media in a reactor that requires coarse it will turn into mush.
You can buy used no problem. Make sure the co2 tank has a good hydro date on it, if not they won't fill it for you until you get it rehydroed. Also make sure the continued works and everything, you would be surprised alot of people expose it to water without even knowing they did (ie. Tank overflows).
People will go left and right about calcium reactor brands so you are honestly better off doing your homework as researching what's best for you.
Goodluck!
