The only explanation for increased FC loss overnight is additional biological load in the pool consuming the chlorine.
Maybe the CYA level wasn't 0 to start with?
Maybe the CYA level wasn't 0 to start with?
When I first started testing I got similar inconsistent results because I was holding the bottle on its side as I squeezed out the drops.
Once I followed the instructions
"Hold the dropper bottles vertically and squeeze gently, so that drops come out slowly and seem to hang on the tip of the dropper bottle for a moment before falling."
I started getting much more consistent results.
Just wanted to share, pool is done and complete. Salt Water is BEAUTIFUL.