What do you guys think about calcium reactors?

I just installed a Coralife a week ago. Still too early to tell how good it is. I have a 120 g tank; the unit is rated for 250 g but it looks like I have to keep it on pretty close to maximum flow to keep the calcium level up.

Whether you need one or not I think depends on the size of the tank and the amount of calcium uptake you have. With my tank it had gotten kind of ridiculous; I was dosing daily with kalkwasser and B-Ionic 2-part buffer system (and going through that pretty quickly), and it was still a chore to maintain the calcium and alkalinity levels. A smaller tank I think would be a lot easier and wouldn't need a calcium reactor.

If you do get one, make sure you get a solenoid valve for the CO2. I found it is almost impossible to maintain a stable CO2 flow (and thus pH) manually.

Tank size will dictate a lot. On tanks 125 gallons and smaller, it is actually cheaper to use a DIY 2 part recipe, believe it or not. However, if you want it "automated" like a reactor, you'll need to buy a doser (automated ones aren't cheap--2 channels are around $250). But you can buy enough mix to last you for a couple years for only $50 or so. When you get much larger than that, even the 2 part will get more expensive and calcium reactors become the only reasonable choice.

Pros--automated system, constantly dosing tank with calcium and alkalinity, therefore balanced. Can provide a lot of both even for larger systems with extensive calcification.

Cons--need CO2, can easily lower tank pH too much if you are careless. Requires more tweaking in the beginning than most methods. Needs a feed pump and mixing pump for chamber.

For my 75, I use DIY 2 part, kalkwasser, all fed via a 3 channel doser. It works well for me and for my size tank, has the potential to keep up with any calcium demand. Much larger, and I'd probably go for a reactor, though.

Amp's advice matches my research locally and online. Even some guys that I consider SPS gurus told me with a small 55g I was better off using a 2 part. Especially since it isn't densely stocked with SPS.

There are a few makes of calcium test kits but I prefer the Salifert ones. And if you really want a comment to confuse you, I don't use a reactor. I have a 182 gal tank and I have never ever dosed with calcium or nything and my calcium levels are high (too high if anything). No idea how I do it but it was the same in my 60 gal tank and I use Reef Crystals for my salt.

I'd still do a 2 part for a 90g.

This article should explain everything for you, if you want to make your own. Commercial alternatives are many, including ESV's B-Ionic, Kent's Tech C-B, TLF's C-Balance, etc.