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.

FC 6.5
CC 0.0
pH 7.3
TA 70
CH 500
CYA 70
Borates 50
Salt 3200

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?

It's probably coming off the walls.

That seems to be my case. The pool was replastered 3 years ago, and based on the service I got from the same pool guy, probably not started properly. So the walls got scale. pH was well off the scale when I started testing, so now all that buildup is finally getting released. I've seen my CH go up 25 (I use the 10ml sample for as high as my CH is) in under a week with no water added but rain.

One thing to remember, -0.35 is higher than -0.6 (-0.60) and therefore you are not in danger. The PC is correct in stating it as less than -0.6, so numbers that would be in danger are -0.7, -0.75, -1.5, etc.

By "coming off of the walls" do you think that the calcium scale is dissolving and increasing the CH? Or is it that the plaster itself is somehow dissolving into the water.

Scale. In my case the white patches are noticeably larger and the scale is getting blotchier.

Constant CH increases usually come from frequently topping off the pool with high CH fill water. When water evaporates, it leaves all the CH behind in the pool. Any CH in the fill water used to top off the pool results in an increase in the CH level. In hot dry parts of the country with high CH fill water this process can be fairly rapid.

HouTex, are you in Houston? Generally our water does not have high CH, but a test of tap water is on order to be sure.

Yes, I am in Houston. I tested my fill water and the CH is 150. During the peak of the drought i was adding about an inch of water a week. I assumed that my highest CH reading of 675 was due to evaporation and repeated topping up. The increase from 410 to 500 in about 10 days occurred with only a very small top up of 150 CH water. I'd like some comfort that the increasing CH is not the result of my plaster being dissolved by the 7.8 to 7.2 swings in pH.

Here are my latest test results. The CH is from a week ago, everything else is from today:

FC 7.0
CC 0.0
TA 80
CH 500
pH 7.3 (added MA today; very close to 7.2 but not quite the same color)
Borates 50
CYA 70
Salt 3200
Temp 90
CSI -.27

After 2-3 days the pH wil drift up to 7.8 (my pool is less than one year old) and the CSI comes up to .12 and then I'll add about 28 oz. of MA and the process repeats. Again, I'd like some comfort that the -.27 to .12 swing is not eating away my plaster.

Just to keep things in perspective. Pure distilled water has a pH of 7 in theory. In reality, it's usually lower because of dissolved CO2 forming carbonic acid. So a pH of 7.2 is unlikely to erode plaster. Rainwater is usually far lower than that, and your patio hasn't dissolved yet...

Given your numbers, you are not dissolving calcium out of the plaster. The PH would need to go below 7.0 before there would be a problem.

Thanks. That's good to know.

My CH is still rising and with our recent rains it's almost certainly not from my fill water. Here are my numbers from this morning:

FC 6
CC 0
pH 7.8 (added MA to lower it down to 7.2)
TA 70
CH 625 (up from 550 a month ago)
CYA 70
Borates 50
Salt 3300

Assuming CSI is useful at my CH level (I'm not sure it is), before adding MA my CSI was .07--which I would think is ok. After adding MA it's down to -.38 (both at 86 degress). I'm still adding about 20-24 oz. of MA every other day.

Over the last month I've hardly had to add tap water (at 150 CH) to my pool--IIRC, two small additions. So my question remains, where is the CH coming from?