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I poor acid in my pool frequently to adjust the pH downward to 7.2-7.8. Chlorine does not work well as a sanitizer when the pH varies from that range. When you tune up pool water you correct problems in this order: 1. Alkalinity (baking soda to increase) 2. pH (hydrochloric acid to decrease) 3. Chlorine (bleach to increase, lots of choices but it is all hypochlorite in a chlorine pool)
Bleach is just the ticket for a heavily used pool, and should be added after sundown to allow it to do its job except for an emergency sanitizer boost similar to what appears to be shown.
So, am I missing some historical atrocity where someone wanted to damage their pool liner and equipment with a tremendous amount of acid?
Chlorine works even better below 7.0, but your plaster and equipment will not be happy.
For the folks at home, not all "chlorine" is the same. Using "pucks" will always lead to problems because it's not just chlorine, but also cyanuric acid (aka 'conditioner'). Cyanuric acid doesn't break down so it builds up in your pool and as the levels get higher the effectiveness of the chlorine is reduced until you reach the tipping point and start having algae and/or cloudy water even though you're doing everything right. The pool boy/store is unlikely to know what's really going on and they have lots of invented terms like "chlorine lock" and products to sell you to fix the problem. The problem is pucks and if you use liquid chlorine you will never have that problem.
I simultaneously siphon and fill the pool to adjust CyA down, but then have to redo alk and ph. I am not disciplined enough to go full-liquid in my sunny area, and too cheap to buy a liquid chlorinator.
These comments are too white. I apologize to OP and the diverse poal community for directing this discussion in such a way as to favor pooled persons over the not-yet-pooled. Let's stay on topic, people, and read the damn side bar. This is not who I am.
I simultaneously siphon and fill the pool to adjust CyA down, but then have to redo alk and ph. I am not disciplined enough to go full-liquid in my sunny area, and too cheap to buy a liquid chlorinator.
You probably wouldn't do that if you lived in an area that charged much for water. In our area a 1/2 drain and fill would cost me about $62. In my pool I would have to do that every few weeks to keep cyanuric acid between 30 and 60 ppm. Each puck adds 3.3 ppm cyanuric acid per 10,000 gallons. My pool is 20,000 gallons, so going from 30 ppm to 60 ppm takes about 26 pucks.
Pool guy here. You are correct.
I'm new working in the pool industry, used to do ponds. Can confirm
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