I mentioned this in my "Algae Diary" post, but I thought it deserved some attention on its own. I opened with algae this year, and my first water test showed a dramatic decrease in TA, but CYA was at about the level of last summer, 35ppm. This puzzled me because if the TA had dropped, that indicated major dilution from rainwater over the winter, but CYA should have been affected the same way.
Now that the water is clear, I retested the CYA and found it to be less than 20. I had suspected that the presence of the algae and dirt from the winter had affected the CYA turbidity test. Waterbear felt that this was the case as well. Apparently that is confirmed. Interestingly, the pool water without the CYA reagent seemed very clear in the CYA test tube, with the dot clearly visible and sharp, even when the tube was full.