The PH, TA, CL in my pool are all within desirable levels. Since I have a outside pool with a power vinyl cover I keep the CYA at less than 20. Because of the cover I keep the TC between 1 and 2. I don't know for sure how much below 20 it is but when I do the test for CYA I can still see an obscured black dot when the tube is full of solution. 5 days ago I noticed a green/yellow stringy stuff in the water. The water is very clear. I presume it might be mustard algae. Therefore I decided to slam the pool. I used the CYA Chart for slamming with a CYA level of 20. It requires that I maintain the slam level at 10. The first night I preformed the overnight FC test. The TC the next morning was 5. I slammed the pool again back to 10. The next morning it was at 5 again. Again I brought the TC back to 10 and the next morning it was back to 5. I measured the CC also that morning and it was 0. Out of frustration and because of my schedule I was not able to slam it again yesterday. This morning the FC was 1. I added enough bleach to bring it back up to 10 and am again trying to get it to take care of whatever is in the pool. All during this time the water has remained clear, the CC is at 0. I have checked it every 4 hours today. At 12;30pm it was down to 7.5. Added bleach to bring it back up to 10. Checked at 4:30 pm - 8.5, added bleach, checked at 7:50-9.0 added bleach. Will now check for loss this night and retest in morning. There was a small amount of the green/yellow stuff on the floor but was not sticking. So I got 2 out of three correct but can not get the FC to not drop more than 1 overnight. Could it be I need to increase the CYA even though I keep the pool coved when not used and always add Bleach after each use. Any suggestions?