I usually open to 0 cya in the spring. I live in the woods, and have a mesh cover, so I'm sure all kinds of things get in my pool, and the cya is eaten up by all that stuff. The same has happened in your pool, since you mentioned that you had a lot of stuff in your pool. You are fighting some kind of biological thing in the water. You will have to keep adding chlorine 3x a day to get it back to shock, keep brushing, backwashing, and keep the filter going 24/7. This year my pool was a pond when I opened - it took me almost 2 weeks of keeping my water at shock level for it to hold overnight. Even though the water was clear, and I didn't get any cc, the chlorine was still being eaten up. It didn't fall as fast the second week, but it still went down a couple of ppms, so I just kept adding bleach. My advice would be to kill it with chlorine now, and really keep on it, this way you will not be fighting it all summer. My pool is sparkling clean, and holding the chlorine! Just when I thought it never would. Let me know how it goes - keep using the trichlor pucks too - it is a good way to get some cya in your water. As for the stain, I think the eraser will work.