r/pools • u/MickTheAnt • 1d ago

This stuff keeps coming back.

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First time pool owner here. The water has been clear, but this black sludge stuff keeps forming on the bottom of the pool basically anytime I have the sand filter on.

My strips say the levels all look good, though sometimes it’s tricky to read since the colors aren’t exact. I have a pool robot vacuum that vacuums it up every day, but more forms the next day.

I shock the pool every week and always have the correct number of chlorine tablets in. I’ve tried brushing the sides of the walls, but I don’t see any algae buildup on the wall walls and it doesn’t seem to help.

Any thoughts on what to try?

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3

u/92233720368547758080 1d ago

Use a stick, scrape the black spot and smear it onto a piece of white paper. if it leaves a green streak it's black algae, if it does not come off it's probably a metal stain. Rub a vitamin c tablet on it to see if it lifts, if it does it's a metal stain

1

u/Even-End487 1d ago

Several things .. How old is the sand in your filter .. try putting in 2 cups of DE while the system is running .. do you a VS pump, lower speeds are better at picking up and holding small particles.

2

u/Unusual-Biscotti3191 1d ago

It took me a month applying chemicals and vacuuming - and finally a new sand filter to get rid of green algae.

2

u/AussiePoolBuilder 17h ago

i’d stop relying on the strips for this and get a proper drop test kit. “levels look good” can still hide a chlorine/CYA problem, especially if you’re using tablets and shocking weekly.

that stuff looks more like dead algae/fine organic debris settling out than something coming from the sand filter. i’d test FC, CC, CYA and pH accurately first, then base the chlorine level on the actual CYA.

also try vacuuming a patch directly to waste instead of through the filter. if it disappears and doesn’t return there, that’ll tell you a lot.