Why is CH Rising?

[HouTex]

Last week I replaced about 40% of my water to reduce my CH. It had been 675 and after the refill it was 380. I tested this morning and CH is 440. I've been trying to lower my TA before topping up the borates from 30 back to 50, so I've been adding MA every day to get my pH back down to 7.2 from 7.5. It takes about 25 oz. of MA each day to lower the pH back down to 7.2 (my pool is still new so pH is constantly rising; the high TA also contributes to the rising pH). Here are my complete test results:

FC 5.0
CC 0
TA 90 (down from about 120 last week and it's getting close to 80)
CH 440
CYA 70
Salt 3100
Borates 30
Temp 90

PC shows that my CSI is -.33. What accounts for the increase in CH? There is some scale on the plaster from when the pH was higher and the CH was in the high 600s. Is the CH from the scale being dissolved? I only added a little water yesterday and my fill water has CH of 150. I can't believe that would have caused it. Any other ideas?

By the way, after getting my TA down to 80 and adding more Borax/MA I plan to keep my pH closer to 7.5 and shoot for a slightly negative CSI.

 

[JasonLion]

If your fill water has CH in it, and you lose water to evaporation and then top off the pool, the CH level in the pool will go up.

It is also possible for CH scale to be dissolved back into the water, but that seems quite unlikely given your test results.

 

[HouTex]

My fill water's CH is 150 and I only added about 1/2-3/4 inch of water yesterday. I did the drain/refill last week and CH was 380 after the refill. Could this one top up in one week possibly cause CH to increase from 380 to 440? That seems like a lot for so little added water.

 

[JasonLion]

With a starting CH level of 675, and a fill water CH level of 150, replacing 40% of your water will lower CH to 465, since none of those numbers are exact having CH at 440 after the water replacement is perfectly plausible. I suspect the 380 test result was made before the fill water had throughly mixed into the pool water.

 

[HouTex]

That makes sense--I guess. I tested the water about 12 hours or so after the refill last week. I assumed that was plenty of time for the water to mix--especially with my oversized pump running 24/7. Thanks.