I respectfully disagree with regard to raising Total Alkalinity (TA) and Calcium Hardness (CH). The current numbers result in a saturation index of a little less than -1.0 and though I know that generally speaking the index is overrated and overrused (or misused) and that one has quite a bit of leeway before there is a problem, I think the water is too aggressive and will leech out or pit the gunite/plaster, at least over time. If you have lowered your pH to 7.5, then you are creating very aggressive water. I don't see any experience on this forum on the corrosive side of the water except for low pH which affected metals, but on the scaling side people usually reported seeing something when the index was > 1.0 (some saw cloudiness at +0.75).
You said that you find that your pH has a tendency to rise (and you are already at 8.0), this could very well be due to your dissolving Calcium Carbonate from your gunite/plaster because that is the effect you would see -- a rise in pH of 0.5 would also raise the TA and CH by about 2 (not measurable). Your TA is quite low so I doubt that you are experiencing any noticeable carbon dioxide outgassing which is the other source of pH rise. If your pool was new or if the repair involved new gunite/plaster, then the pH rise could be due to curing of the new gunite/plaster.
If I were you, I would add both calcium chloride to raise the CH and sodium bicarbonate (baking soda) to raise the TA. Even if you added half of the recommended amounts to get to a TA of 50 and a CH of 150, that would still be a lot better than the situation you are in now. You can then see if your pH has the strong tendency to rise or not (the higher TA will cut down that tendency, but the key is how much acid you need to add over time to maintain pH). If your pH still has a strong tendency to rise, then this is likely the curing of the new plaster/gunite and will take months to subside (you'll just have to stock up on acid).
Richard
Bookmarks