Black mold in a pool grows on surfaces where water sits still, usually on the walls, floor, or around the waterline, and you can remove it with a pool brush, chlorine shock treatment, and algaecide.

Black mold (actually a dark algae, not true mold) appears as black or dark green spots and streaks on pool surfaces. It clings to plaster, vinyl, or concrete and does not wash away with normal circulation. The fastest way to clear it is a combination of three steps: brush the affected area hard to break the algae's grip, add a chlorine shock treatment to kill it, and follow with an algaecide to prevent regrowth.

The whole process takes one to three days depending on how much mold is present and your pool's size. You will need to brush daily during treatment and keep the pump running continuously. Most pools return to clear water within 48 hours if you follow the steps in order.

Key Takeaways

  • Black mold clings to pool surfaces and requires vigorous brushing to break its hold before chemical treatment will work.
  • Chlorine shock treatment kills the mold, but you must brush first or the chemicals cannot reach all of it.
  • Algaecide applied after shock treatment prevents the mold from returning within days.
  • Keep your pump and filter running continuously during treatment to circulate chemicals and remove dead algae.
  • Test your chlorine level daily and maintain it at 2 to 4 parts per million once the mold is gone.

Brush the Pool Surfaces Hard and Often

Start by brushing every surface where you see black mold—walls, floor, steps, and the waterline. Use a stiff pool brush (not a soft one) and scrub in circular motions, pressing hard. Black mold has a waxy coating that protects it, so gentle brushing will not work. Spend at least 10 to 15 minutes on each affected area, even if the mold does not disappear during brushing. Your goal is to break the algae's grip so that chemicals can penetrate.

Brush again the next morning and again in the evening. Repeat this twice-daily brushing for the first two days of treatment. The mold will begin to loosen and cloud the water—this is normal and means the treatment is working. Do not stop brushing just because the water looks cloudy; the cloudiness is dead algae that the filter will remove.

Add Chlorine Shock Treatment

After your first brushing session, add a chlorine shock treatment to the pool. Shock is a high-dose chlorine product that kills algae and bacteria quickly. The amount you need depends on your pool's volume in gallons—check your pool's documentation or measure the length, width, and average depth, then multiply those three numbers together. Most shock products come with dosing instructions on the bag or bucket.

Add the shock in the evening after brushing, when the sun is lower and chlorine will not break down as fast. Pour it around the perimeter of the pool while the pump is running. Do not add shock and then turn the pump on—the chemicals need circulation to work. Run the pump continuously for at least 24 hours after adding shock. Test the chlorine level the next morning with a test kit; it should read between 5 and 10 parts per million. If it is lower, add another dose of shock.

Use Algaecide to Stop Regrowth

Once the chlorine level has dropped back to 2 to 4 parts per million (usually by the second morning), add an algaecide. Algaecide is a separate chemical from chlorine and works by coating algae cells so they cannot reproduce. It is especially important for black mold because this type of algae regrows quickly if you do not use it.

Follow the dosing instructions on the algaecide bottle based on your pool volume. Add it while the pump is running, just as you did with shock. Let the pump run for another 24 hours. The water should clear noticeably by this point. If you still see black spots after two full days of treatment, brush those areas again and add a second dose of algaecide.

Keep the Pump Running and Filter Clean

During the entire treatment period, run your pump and filter continuously—do not turn them off at night. The circulation spreads chemicals throughout the pool and pushes dead algae toward the filter. A stationary pool will not clear, even with chemicals added.

Check your filter daily. Black mold treatment will load the filter with dead algae, and a clogged filter slows circulation. If the filter pressure gauge reads higher than normal, backwash the filter (if you have a sand or DE filter) or clean the cartridge (if you have a cartridge filter). Backwashing takes 3 to 5 minutes; cleaning a cartridge takes 10 to 15 minutes with a garden hose. Do this once or twice daily during treatment.

Test and Maintain Chlorine Levels

Use a chlorine test kit (available at any pool supply store) to check your chlorine level each morning during treatment and once daily after the mold is gone. During shock treatment, the level will be high (5 to 10 ppm). As the days pass, it will drop naturally. Once it reaches 2 to 4 ppm and the water is clear, you have finished treatment.

After the mold is gone, maintain your chlorine at 2 to 4 ppm by adding small amounts of chlorine tablets or liquid chlorine once or twice a week, depending on sunlight and pool use. This prevents black mold from returning. If you notice black spots appearing again within a few weeks, brush and shock when ready—a second treatment usually clears it faster than the first.

Common Mistakes That Slow Clearing

The most common mistake is not brushing hard enough or often enough. Black mold will not respond to chemicals alone; you must break its surface first. Brushing only once and then waiting for chemicals to work will extend treatment to five or seven days instead of two or three.

The second mistake is turning off the pump at night to save electricity. The pump must run continuously for chemicals to circulate and dead algae to be filtered out. Turning it off resets your progress.

A third mistake is adding shock and algaecide at the same time. Shock is a very high chlorine dose, and algaecide works best when chlorine is lower. Add shock first, wait for the chlorine level to drop to 2 to 4 ppm, then add algaecide. Adding them together wastes the algaecide.

Frequently Asked Questions

Is black mold in a pool dangerous to swim in?

Black mold itself is not toxic, but it indicates poor water chemistry and circulation. Pools with visible mold often have other bacteria and algae present that can cause skin or ear infections. Do not swim until the water is clear and chlorine is at the correct level.

Can I use just bleach instead of pool shock?

Household bleach is diluted chlorine and will not raise the chlorine level fast enough to kill black mold quickly. Pool shock is concentrated and designed for this purpose. Bleach also adds sodium, which builds up in the pool over time. Use pool shock for faster results.

How long does it take to see results?

The water will begin to cloud within 12 hours of shock treatment, which means dead algae is circulating. Visible black spots usually fade within 24 to 48 hours if you brush twice daily and keep the pump running. Complete clearing takes two to three days for most pools.

Will black mold come back after I treat it?

It can return if chlorine drops too low or the pump stops running for extended periods. Maintain chlorine at 2 to 4 ppm and run the pump at least 8 to 12 hours daily to prevent regrowth. Brush the pool weekly to catch any new spots early.

What if the mold is only in one corner or on one wall?

Brush that area hard and often, but you still need to shock the entire pool. Mold spores circulate in the water, and shock treatment prevents them from settling elsewhere. Treating only the visible spot will leave spores in the water that regrow within days.