In my black algae thread one person mentioned my pool shop CH levels being suspect because they were less in a second test done a few weeks afterwards, they said CH would only fall if you drained the water.
Got my TF-100 test kit and tested CH, was 300. I did this twice to make sure testing was right. Today 4 days later, it's 250 - again tested twice and exactly the same. How could it drop almost 20% with no change to water?
We had maybe 10-15 mins of heavyish rain yesterday but that prob added 1inch of water max. As a % refill / dilution it would be like 2-5% or less.