It's all going to depend on your tank, what you're keeping and how things are going.
IMO, any tank will benefit from "reactors" running carbon and phosban (or some other
GFO). I put reactors in quotes here since they're not technically reactors, but I digress. Calcium reactors use tank water and CO2 gas to slowly disolve calcium media that helps keep your calcium and alkalinity levels up. There's not really a need for one unless you have a high calcium demand (Lots of SPS, lots of clams, etc.) Even in those cases, it's certainly possible to keep up without one. I have a fairly high calcium demand, and I use calcium and alkalinity solutions dosed into the tank. Generaly speaking, they're expensive to purchase, can be difficult to tune just right, and usualy require a
pH controller to run things. A
nitrate reactor can help lower
nitrate levels in heavily stocked tanks. They're even more finiky than calcium reactors, are expensive and some require special sulfur media and/or feeding of sugar or alcohol. The vast majority of people can keep their
nitrates to an acceptable level with lower bio-loads, good tank cleanings, water changes and natural nutrient exports via
macroalgae in a
sump or
refugium. I've never heard of a
nitrite reactor, so I can't help you much there.
