OK.

Let's do this.


Buy a cheap OTO (yellow drops) chlorine test. On your pool, an 10 gallon dose of *plain* 6% household bleach will be about 15 ppm chlorine.
Add 10 gallons within 2 hours of sunset; test 1 hour later, again within 2 hours of sunrise, and again that evening.

The first test will verify your test method; the 2nd will see if you have a chlorine demand apart from sunlight, and the third will check to see if you really are blocking 90% of the UV.

Plus, 15 ppm is enough to do some good with your algae, if your CYA test results are correct.

Ben

PS. Consider getting a DPD-FAS testkit. It would make it much easier to tell what's going on, because it is the ONLY way to accurately measure chlorine levels in the 0 - 20 ppm range. (Link to Amazon sold kits below; wait a day if "Amato Ind" is not the seller; other Amazon sellers sometimes confuse the K2006 with the non-FAS-DPD K2005)