Tks so much... really helpful. I have to own up and tell you I live in Bali. I have a tiny tiled concrete pool (about 10,000 ltr) but it's been a nightmare trying to keep it clear of algae. My major problem (besides trying to communicate technical terms and chemicals in the local language) is there are no decent test kits here, just the basic. So what I'm going to try is to shock it to the max level (FC 10 - its relatively new water and never been properly shocked before so CYA 'should be low' ), if that turns from green to light blue or white then continue to keep it at that level for a few days then flock it and start again. Of course the Ph and TA will be checked (the TA is low - about 60 however finding baking soda here except in 50mg containers is a challenge), the sand in the filter is new and I'll be constantly brushing the pool while shocking it. Wish me luck!