What kit did you use to get your numbers? CYA tests top out at 100 so a reading of 100 can mean 100 or any number higher than 100. Redo the test with a dilution. Take equal parts of pool water and distilled water and then run the CYA test again with that mixture and multiply your results by 2. Then, post that reading.

I'm not too familiar with Yellow Out, but there was a fairly recent thread where PoolDoc gave some information and advice in regards to using it. From what he told the other poster, you want your pH high while using it.

I can't link the thread here because as a trial member, you wouldn't be able to follow the link. But, if you will first log out, you'll be able to see and read the rest of the forum. Look in the section of the forum called "Dealing With Algae and Slime." I bumped up the thread I want you to read that is titled "Yellow Mustard Algae and Yellow Out."

Welcome to the Pool Forum, by the way!