What Are the Black Spots on My Pool Plaster?
Black spots on pool plaster are usually black algae. It grows in small, dark, raised dots, often in rough or shady areas, and it roots into the plaster under a protective coating, so normal chlorine levels do not kill it. To get rid of it, scrub the spots with a stainless steel brush, treat them directly with chlorine, and shock the pool, usually more than once. When black algae covers a lot of the pool, a chlorine wash is the faster fix.
Is it black algae or a stain?
- Black algae. Raised dots or patches that feel slightly rough or slimy. Scraping one leaves a dark smear, and the spots slowly spread.
- Metal stain. Flat, with no texture. It does not brush off or spread the way algae does.
- Leaf or debris stain. A brown or dark mark in the shape of whatever sat there. It often fades with chlorine and sunlight.
- Not sure? Bring us a photo along with a water sample.
How do I get rid of black algae?
- Step 1: Balance the water. Bring pH to 7.2 to 7.4 so chlorine works harder.
- Step 2: Scrub every spot hard with a stainless steel brush. This breaks the coating that protects the algae. Use stainless steel on bare plaster only, never on vinyl, fiberglass, paint, or pebble.
- Step 3: Treat the spots directly. On plaster, you can rub a chlorine tablet on each spot while wearing chemical-resistant gloves.
- Step 4: Shock the pool in the evening and hold chlorine high. Liquid chlorine adds no calcium or stabilizer.
- Step 5: Ask us about an algaecide made for black algae.
- Step 6: Brush daily and keep chlorine high for several days. Clean the filter when you finish.
- Step 7: Repeat if spots return. Black algae often takes more than one round.
Why does black algae come back?
The roots sit below the surface of the plaster, so a treatment that only kills the top layer leaves the rest to grow back. It also rides in on swimsuits, toys, and pool equipment that have been in lakes, rivers, or another pool with algae. Older, rougher plaster is most at risk, and Bakersfield's long warm season gives it months to spread. Keeping chlorine steady and brushing weekly is the best prevention.
When is a chlorine wash the better fix?
When black algae covers large areas or keeps returning after treatment, chemicals alone are slow. For a chlorine wash, we drain the pool, spray every surface with straight HASA liquid chlorine, and pressure wash until it is clean. Then we clean the filter, refill, and add startup chemicals. Call (661) 664-9000 for a quote.
Is black algae harmful to swimmers?
Black algae is not usually dangerous by itself, but it can shelter bacteria, and it means the pool has not been sanitizing well. Treat it before swimming regularly.
Can I use a wire brush on my pool?
Only on bare plaster. A stainless steel brush will damage vinyl, fiberglass, painted, and pebble surfaces. Use a nylon brush on those.
Will shock alone kill black algae?
Usually not. The coating on black algae protects it. You have to brush hard first so the chlorine can reach it.
How long does it take to get rid of black algae?
Plan on one to two weeks of brushing and high chlorine for a light case, and expect to repeat the treatment. Heavy cases are faster to fix with a chlorine wash.
Don't cry, we can help
Bring a water sample to Top Dog Pool Supply. Local pool owners get a free water test on our Taylor Xpress Flex system, and we'll tell you what to add, and how much, for your pool's size.
Visit us at 7440 District Blvd Suite B, Bakersfield, CA 93313, or call (661) 664-9000. We're open Monday to Friday 9 to 5, Saturday 9 to 4, and Sundays 10 to 4 from April through September.