Strange stains appearing in my pool - any ideas?


Apr 13, 2020
Los Angeles
Relatively new pool owner here. Full disclosure: I am maintaining the pool myself and do not have a full handle on stable chemistry yet. Having said that, apart from a bit of algae in the summer I have not had too many problems....until now.

A few months ago I started to notice these rust-colored stains appearing at the bottom of the pool. For the most part they are in clusters which makes me think it might be some sort of fungus or algae. They do not scrub off.

The elongated stains in the clusters are about an inch long each.

Apart from the clusters there are also some small, isolated circular stains appearing of similar rusty color.

We do occasionally get bird droppings and wind-blown plant debris but not too much, and certainly nothing that is new over the past couple of months.

So first question is what is causing the stains, and then what can I do to (a) get rid of them and (b) stop them happening again?



Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
Laughlin, NV
They appear to be rust.

Get some Vitamin C tablets. Put a hand full in a thin sock. Crush them up a bit. hold the sock against the stain.

If that eliminates the stain, it is iron. From what, well, could be fertilizer. Could be wire ties from the rebar close to the surface.

If the stains begin to reappear, use a strong magnet on them to see if there is iron there.


Gold Supporter
TFP Guide
Jul 21, 2013
Northern NJ

They look like iron stains. Anyone around your house doing roofing or siding work?


Apr 13, 2020
Los Angeles
Could it be a single nail that has been rolling around a bit, perhaps as the robot passes over? That would explain it! Yes - we have had some contractor work recently and nails ended up all over the Dang place.
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