A few days ago, I started draining some water to lower my CYA level.
See this post:
Anyway, at the start of the drain, here's the readings from the TF100 kit
Note, I did 3 tests of everything before I started draining and TA, CH, CYA were the same for all 3 tests, pH and FC changed because of addition of bleach and acid.
pH - 7.5
FC - 16
CC - 0
TA - 170
CH - 300
CYA - 55
So I drained and refilled 700 gallons (2.5%) and did the pH, FC test - ph - 7.5, FC - 14
Next day, another 700 gallons, pH - 7.4, FC - 14
Next day, another 700 gallones, pH - 7.4, FC 13.5
Then I didn't do anything for 3 days, no draining, no acid, nothing
Yesterday I measured everything:
pH - 7.4
FC - 13
CC - 0
TA - 100
CH - 250
CYA - 40
And today, I measured it again and got pretty much the same readings except pH was 7.5.
So how could draining and adding 7.5% water cause the CYA to drop from 55 to 40,
CH to drop from 300 to 250 and
TA to drop from 170 to 100?