[Myka]

You don't necessarily have to get a calcium reactor with a kalk reactor attached to it, but it isn't a bad idea. There are pros and cons to having one piece of equipment rather than two. IMO, it is a good plan to use kalk from the get go. I prefer to dose kalk using a doser rather than putting in the ATO because it's easier to control (especially in new tanks or those with lower demand), but that's up to you. I also like to add vinegar to the kalk vessel because then you can mix it to 3 tsp per gallon rather than just 2 tsp per gallon plus it has the added benefit of carbon dosing, and you can't do that if you use a kalk reactor. I love kalk - it has many benefits, and helping with pH is only one. I think you're on the right track by planning to use it. Also, if you feed your CaX effluent into the skimmer intake this will help to dissipate the CO2 (which is what lowers the pH). Do use a pH controller too, not just a monitor.

On the other hand, you could choose a cheaper 2-part to dose. Both the Fauna Marin and Tropic Marin systems are cheaper. They are "balling" systems which include the option to dose trace elements with along with the "big 3", but you can just use the 3 powders instead. Any powdered product will be cheaper than any solution product simply because of the concentration and the cost of shipping water.