How to keep Ca, Alk stable

[snowfish]

I finally got the Alk and Ca to where I want them, but how do I keep them there. I have test kits, and am going broke testing testing testing :x The tank is 75 gal, sps, softies, and 5 fish. I do a 20% water change about every 2 weeks. I do not have an RODI unit, and so I use purified tap water to mix with Reef Crystals. Please give me some suggestions for stability.

 

[Fishie Nut]

Randy's 2-part
Ca Reactor