Not sure it's a stain.. but it won't brush

[mphare]

Thanks everyone.

Now that I've got some salt in the pool, and the SWG is making chlorine.. I'm seeing less of the buildup.

The concentrated CL on the affected area seems like the next step.

While it doesn't feel organic (I expect organic to feel slippery, not gritty, but I may be wrong about that) the fact that now that I have CL the stain is less, seems to point in that direction.

[Big_D]

Based on your information you probably have some type of algae which is probably not in the substrate yet at least. Good for you and I bet you are right it is more of a chemical maintenance issue. I also agree you can add some regular old bleach and clear up most of those. My old pool would get blooms in the summer since the circulation was poor. Back then I did not know better and I would buy that expensive shock to kill it.

On the new pool I also have a SWG but I will dump in a gallon on occasion to make sure those bad organisms are in check. Also living here in Texas that summer heat is rough on Chlorine and pool use is high so it is too cheap not to stay ahead of the game.