I have had an encroaching cyano problem which I have dealt with in all the traditional methods. I replaced all my ro/di components as well.
Eventually, I measured the alkalinity...
It didn't surprise me that the 120g (after removal of the sandbed) suffered a bit. It did surprise me that all the other tanks still had low alk after totally redoing the ro/di.
So tonite, I measured my mixed SW (mixed for a while). Kent salt which has done us well for over four years, and my my alk has been on the bingman equilibrium with my Ca (which is over 400). The older mixed sw was at 2.2. I figured something was up with the old plastic container. So I mixed a fresh batch at 1.026 - 76F and it read 2.6. Lower than NSW.
I have been buffering the 120g, never occurred to me it was a source problem. Is this something new?
BTW, test kits were the old ones, salifert, and the brand new ones, salifert, measured exactly the same.....good review for salifert Kh and Ca