[Delphinus]

Interesting question! Also considering that there are different ways to view what consitutes a "good investment". At any rate I voted calcium reactor and phosban reactor because those were the only two items on the list that I actually have. I probably didn't need to vote phosban reactor because I'm not sure that it's neither unnecessary (it could be viewed as necessary), nor a good investment (eh, I've been running carbon in them for a while, I only just started up one with actual phosphate reducing media two days ago. Hardly a lifesaver in my case!).

Calcium reactor on the other hand ... sure, you could argue it's not necessary, but I find it's one of the best bangs-for-buck there is. Not only do I find running a calcium reactor cheaper in the long run over other methods (although the startup costs are high, if you look at the cost of additives over 2 or 3 years you might find the additives to be the more costly option. At least I did at the rate I was going through additives), but also I find there's just nothing to compare to the set-and-forget aspect. It was a little tricky trying to dial them in at first but once they're set, they're set. If I go away for a weekend or whatever, I don't have to get someone to add my alkalinity additive in the morning and the calcium additive at night (because you shouldn't add them at the same time), or alternatively not worry about making my tank go through weird swings because nobody's adding them. So the peace of mind counts as a return on that investment as well. And, and and and!!! One more thing, I found that I got *better* growth of SPS using a calcium reactor than I was getting with the additives, despite even having lower Ca and Alk values than I was acheiving with the additives. My thinking on this is that the values are replenished at the same rate they are depleted, thus acheiving a more constant steady-state situation, whereas you get a "sawtooth wave" effect with the additives (sudden rise when you add them, followed by slow depletion over the course of a day, rinse and repeat). So ... cost, peace of mind, better results. Complete slam dunk!