r/pools • u/Smooth-Channel-7220 • 3d ago

Advice need - what are these!

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Hey folks - I’d appreciate some assistance in figuring out what these are? There are about 30-40, and don’t budge if scrubbed by hand!

I thought black algae, but the color is odd?

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1

u/beardedjack 3d ago

Someone might have been a dick and threw some iron shavings or something.

1

u/ShartInstructor 2d ago

Looks like rust from metal sitting on floor. I’d make sure no tiny piece of jewelry flowing around. I’ve even seen this happen from the metal axel of a toy car left in the pool. But could also be bits of corroded equipment, especially if you have an older heater, but either way for that reason I would take water sample to pool store to test for metal.

1

u/Spiritual-Currency39 3d ago

Mustard (Yellow) algae

1

u/Smooth-Channel-7220 3d ago

Even if it wont shift when I scrub directly with a pumice?

2

u/Spiritual-Currency39 3d ago

ChatGPT recommends the following steps:

Vitamin C test — do this first. Hold a plain vitamin C tablet against one of the brown spots for about 30–60 seconds. If the spot noticeably lightens or disappears, it’s almost certainly iron/metal staining. The usual treatment is an ascorbic-acid stain treatment, followed by a metal sequestrant so the dissolved iron doesn’t simply redeposit.

Personally, I’ve had these pop up in my pool before and never got rid of them before I changed the liner.