Recurring black dust

[aylad]

It's possible that it's mustard algae, in which case it will grow on the bottom and up the sides of the pool. If that's it, then you'll need very high shock levels sustained for a longer period of time to get rid of it. In order to know how much chlorine you need to sustain, you really need to know your CYA level. If you're using algaecide, then you're wasting your time to shock first, since the algaecide will deplete your chlorine rather quickly. As algaecide is much better at preventing algae than killing it, I would wait on the algaecide and just sustain a shock on the pool until you're no longer losing any chlorine at night. If you'll measure your chlorine at night after the sun is gone, and measure it again in the morning before the sun hits it, you'll get an idea of whether there's any chlorine loss in that time period, and if so, you know it's something in the water that needs to be killed. Sustain your chlorine at shock level (again, that's dependent on your CYA level) until you no longer lose chlorine overnight.

If you're not losing any chlorine overnight, and if it is only on the bottom and not the sides, then I would suspect either pollen or sand that has blown in and sunk. The only thing you can really do for pollen is stir it up really well into your water and use a slime bag on the basket to catch it. Might also want to check and see if it's sand, maybe being blown back into the pool from a broken lateral in your filter.

Hope this helps!
Janet