What is considered normal for an overnight FC drop? I am trying to eliminate mustard algae as a problem and tested my water last night and again this morning. I tested last night(more than once) and got 4.5 and 5 in 2 different tests. This morning I am getting 4 and 4.5 in 3 different tests. Looks like I have lost .5ppm FC and possibly up to 1 ppm but not more than 1ppm. I was hoping for no loss at all. Is this drop normal?