Actually, Rich, you do give me a clue. The CYA measured around 70 before I left on vacation, was 40 when I first measured it with the bloom, with an FC of 16.5, then measured 60 with an FC of 19. In both cases the water was, of course, cloudy.

Could the algae itself, still present in the first test, but not in the second, have an effect on the reading?