Learn from my mistake. Ph dropped from 7.9 to 6.8 overnight. Wtf?


Nov 15, 2019
Frisco, TX
Pool Size
Liquid Chlorine
Tuesday was a busy morning. Had a window crew here. Stone repair crew for outdoor kitchen. And a trim crew for our new cedar patio cover.

I was done testing my water and needed to bump ph down from 7.9 to 7.6 per pool math calc I needed 11 ounces. No big deal. Poured it into my pre-marked cup and proceed to the return and start slowly adding it. Then I get distracted. Every crew had a question for me. I finish pouring the ma but don't rinse the cup in the pool ( I figured that out today).

The next morning, I grab my water cup for testing, and fill it. Yes it is the same one for my ma pours. Run my usual tests and woah, ph is 6.8. Same sample water tested again and it's 6.8. My csi dropped to -1.03. Oh know. Run the pool to aerate all day until 9 pm. Double check that my ma and chlorine are the right bottles. Yep. Didn't add to much ma so what happened?

Go out and test this morning and ph is 7.6. Alright. Check again and all good.

My deduction is that there was ma residue in the water sample cup I regularly use for testing because I didn't rinse it the day before. The residual ma bumped my ph test down to 6.8. Lesson learned.

Does that make sense?


Gold Supporter
TFP Guide
Jul 21, 2013
Northern NJ
Pool Size
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60

It is important to keep the cups and vials you use for testing clean and not used for other purposes. Any residual chemicals can skew your test results.

You should have a dedicated cup for measuring acid and a different one for measuring chlorine. Best to never mix any chemicals in the same cups.
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