Pool Troubleshooting /
What Does Black Algae Look Like in a Pool?
Black algae usually shows up as stubborn black or dark blue-green spots on pool walls, steps, and corners that look stained, rough, and hard to brush away.
Archive date: this article was written for PoolSkimmer on Oct 23, 2026 and given the earlier archive date of Jun 30, 2026 so the blog reads as an organized archive. It was not published on that date.
Identifying black algae in the pool
Black algae tends to present itself as small spots of black, dark gray or a deep blue-green that are firmly attached to hard surfaces. Rather than looking like loose dirt, it is more akin to an embedded patch or stain on the plaster, concrete, tile, steps and walls. Even with water flowing by, the spots will not budge.
Visual indicators
There are a number of telltale signs:
- Round or oddly shaped black marks on the surface
- Patches with a rough, pitted or grainy appearance
- Dark spots that don’t have the tendency to drift or settle as debris does
- Any growth in areas with poor circulation such as corners, ladders and fixtures
Left to its own devices, a few isolated spots can coalesce into a larger cluster.
Distinguishing it from stains and dirt
It is easy to mistake black algae for metal staining or common dirt, particularly if the pool is discolored or the water is anything but clear. To be sure, try this:
1. Put the spot under bright light. 2. Determine if it is sitting on top of the surface or seems part of it. 3. Give it a brush and see if you can alter its shape or just nudge the edges. 4. Look for other similar spots along the walls and in the corners.
Unlike loose debris which will lift or move about, black algae is stubborn and fixed in place.
Where to look
Your first port of call should be any area where the water doesn’t circulate well. Check the pool walls, the waterline, shaded spots and any porous surfaces like plaster. You will also want to inspect the steps, benches and ladders; the algae can easily hide in the texture and pits of these surfaces.
If you think you have black algae
Don’t be quick to dismiss it. And if there is any question as to the safety of the water, put off swimming until you have the matter sorted out.
Turn off the pump before opening, inspecting or cleaning the skimmer basket, and follow the equipment manual for inspection and cleaning. Wear appropriate PPE, follow chemical labels and manufacturer directions, and never mix pool chemicals. Never reach into operating suction equipment or attempt electrical repairs; leave electrical work to a qualified professional.
Time to get a professional?
If the spots come back after you have done your cleaning, or they begin to spread, it is time to make a call. A qualified pool pro can be of assistance if you are unsure whether you are looking at black algae, a stain or some form of damage. They can also advise on the best course of action for your pool care routine in the event of a filtration or water-quality concern.
Sources and further reading
- [Hayward: Weekly pool maintenance](https://www.hayward.com/maintenance/weekly-pool-maintenance)
- [Pentair U-3 skimmer manual: suction safety warnings](https://www.pentair.com/content/dam/extranet/nam/pentair-pool/pool-manuals/u-3-skimmers/395010028C.pdf)
- [SwimmingPool.com: Cleaning the skimmer basket](https://www.swimmingpool.com/maintenance/pump&filter/cleaning-the-skimmer&pump-basket)
- [CDC: Use pool chemicals safely](https://www.cdc.gov/healthy-swimming/media/pdfs/Pool-Chemical-Safety-USE-poster-p.pdf)
- [CPSC: Electrical safety in and around pools](https://www.cpsc.gov/safety-education/safety-guides/pools-and-spas/dont-swim-shocks-electrical-safety-and-around-pools)