Wanted: Mustard algae or suspected mustard algae

[chem geek]

good to know, i jacked my FC to about 20 for 4 days. Seems on now, but this happened before. If my CYA is 70 I would need to leave my FC at 10.5?

Yes. If you don't use an algaecide or completely get rid of the algae via intensive shocking (42 ppm after first lowering the pH to 7.1, but that seems almost insane, doesn't it?), you'd need to keep a minimum of 10.5 ppm FC with your 70 ppm CYA to keep away the yellow/mustard algae. At least that's the experience of users so far. Though technically shocking at 42 ppm FC at 70 ppm CYA is equivalent in disinfecting chlorine to 1.6 ppm FC with no CYA (which is lower than most indoor pools), the high numbers scare me, even though I know the chemistry behind them.