I recently replaced about 40% of my water to lower my calcium from the high 600 range to the low 400 range. After the replacement my TF-100 showed CH at 410 earlier this month. Today the CH test was up to 500. We've had some rain lately and I've only added water twice in the three-four weeks since the water replacement. I probably added 2" worth of water in that time and my fill water is 150. Using the Pool Calculator it would take replacing 26% of my water to go from 500 CH down to 410. I'm not sure the reverse is true (i.e. adding 26% of new 150 CH water to go from 410 CH water to 500 CH water), but in any event I can't believe the two small top ups could increase my CH from 410 to 500.
My pH has been ranging from 7.8 down to 7.2 (the pool is about 7.5 months old) after I add MA. I add about 24 oz or so of MA every other day when pH rises to 7.8 and it brings it down to around 7.2. Today, after I added MA the shade was not quite at the 7.2 level but it was close--hence the 7.3 estimate. Tomorrow the pH will likely be right at 7.5 and then at or very close to 7.8 on Sunday. Here are my full results from this morning.
So where is the CH coming from? Is it from the plaster being corroded? The CSI is -.35 and the PC says "potential to become corrosive to plaster."
I started an earlier thread when my CH was very high and I was concerned about scaling due to the constantly rising pH--and I think I have some scaling, and the general advice was to never let the pH rise above 7.5 (at that CH level). Now I'm concerned about the plaster being corroded. I do notice a some small rough spots on the bottom of the pool that are small gouges in the plaster.
Again, the question is where is the CH coming from, and should I be doing anything differently?