Man Made Live Rock
These are devices that are used to maintain proper Calcium, Alkalinity, and PH of your salt-water tank. Notice I said maintain, not elevate. Although I am sure some use them for that, they function better as maintainers rather than elevators.
Cost is a factor however, while the up front expense is real high the long-term expense is much lower than additives. Many feel they are not needed unless you have hard corals and/or clams. My opinion is that they can still be used to keep your tanks Calcium, Alkalinity, and PH in check. There are also notes of using a Kalk reactor in conjunction with a calcium reactor. This may have merit for some tanks, but certainly not all would require such additions. By the same hand many a soft coral tank wouldn't need a calcium reactor either.
Reactors operate by using Carbon Dioxide gas to lower the PH of the salt-water inside the unit to the point that it easily dissolves the "media". The media is generally large grained calcium-based sand. In simple terms a special grade of the same sand you want to have as the substrate in your tank.
Many people seem to like only one or two of the brands available. I will not mention their names but a search of the net should point you to those they like and why. They also mention that buying the "kits" usually results in buying an inferior needle valve assembly. Which I mention as this is the one part that primarily effects the devices ability to do itís job without needing to tweak it regularly.
Other things people mention are that they are all but useless without a PH Monitor (a PH controller would even be better yet) and can also be rather difficult to dial in. I have found neither of these to be true. If one is using them as maintainers and not elevaters then a regular PH test kit should suffice in my opinion. Note: Do not try to dial one in with the effluent going into your tank. Feed the output hose to a container until you have it set, then feed it to your tank. Preferably the feed should go into a high flow area near the beginning of your sump. I also have not had any trouble dialing in the one I have so I can't complain about any difficulty doing so.
After months of fighting with myself I finally broke down and purchased a Korallin 1502 package. I have been pounding myself in the head since then for not doing it sooner. What a difference in maintenance time!!
I choose the Korallin for the safety shut off feature and am glad I did. Some people complain about it building up CO2, but to me that is much better than dosing raw CO2 to the tank. The few seconds it takes to bleed it off every couple of days is minor time taken compared to the manual dosing from before. The user manual suggests a different setup method to avoid the build-up, but it didn't seem near as bulletproof.