PH - Per Hydrogen, is a measurment system of what elements and minerals have bonded with the H20. Depending on which elements have the highest ratio of molecules to hydrogen ions - your water is either Acidic ( 1-6.9) Base ( 7) Alkyline ( 7.1- 10). We do water changes weekly to replenish the 02 and Hydrogen atoms. Salt mixes add to the Alkalinity as well as the leeching from Rocks and sand. However the molecules that make water Alkyline are used/consumed by Fish, Corals, and Inverts. One such element is Calcium, another Magnesium, yet another Sodium. Stoney corals and coraline algae most especially use this stuff up like a bat outta hell. I know ppl with LPS tanks that add baking soda mixes daily to keep the alkaline levels stable (not dropping) - Your daily R/O water top offs contain none of these Alkyline elements only the salt. And not all salt mixes are created equal when it comes to Alk.
