I had a two little fishies kalkwasser reactor on my old 75g and it performed perfectly. If you are going to do it, I suggest getting one of those. It does come with a pretty good check valve that prevents the backflow that Bang is talking about. It doesn't prevent it all, but it does a good job. Since the reactors outlet doesn't touch the tank water in any way, it does form a shell on the top, but when the pump turns on, it pushes water into the reactor which causes the water level to rise and it just pushes the shell up and releases the saturated limewater into the tank, very, very, slowly. I wouldn't put a kalk reactor on anything less than 90 gallons and if you had room. For my little 29g tank, it's just unnecessary. Then again, if my tank fills up with SPS corals, I may try it out again, since I have the old reactor in storage and I know I can fit a 10g reservoir underneath my tank. Meh, we will see.
I already had to do some tank maintenance this morning, the water level got pretty low. It seems like this tank is evaporating pretty dang fast, but it is next to the air conditioner unit, so there is a lot of air draw past it.
I'm checking another sps frag website that I like, and I can get 6 SPS frags for $95 with free shipping... which I think is a pretty good deal. I just am unsure if I want to spend that money or save it for an emergency. After all, I'll be a daddy again in about a month. whew!