Black algae can turn a sparkling pool into a stubborn mess of dark patches that cling to walls, floors, and steps. If you’ve struggled with those tiny, raised black spots and wondered how to get rid of black algae in pools, this guide’s for you. We’ll define exactly what black algae are and why they appear, then walk through detailed steps on how to remove black algae from pools.
Black algae (also called Pithophora or blue-green algae) are microscopic aquatic organisms and a type of cyanobacteria. In pools, they appear as clumps or dots of mold. Typically, black algae in a pool will appear as small black spots on the floor or walls, sometimes with a blue-green tint.
Black algae thrive on porous or rough surfaces (concrete, gunite, or plaster pools) because they send root-like tendrils into cracks. They can grow in both sunny and shaded areas of a pool. In fiberglass or vinyl pools, it’s rare that they grow; the organism needs something to grab onto.
Black algae in pools usually appear when water chemistry or circulation isn’t ideal. In general, algae spores can enter your pool via wind, rain, or even swimmers — lake or ocean swimsuits can carry them.
These spores need the right conditions to grow. Poor chemical balance is a significant factor. Experts note that black algae thrive when pH and alkalinity levels are too high and sanitizer levels are too low. Maintaining a pH level between 7.2 and 7.6 is recommended. Similarly, if chlorine levels drop, algae can take hold.
Black algae prefer quiet, shaded spots like behind ladders, in floor corners, or in crevices where the pump flow is weakest. If the pump isn’t running enough or the filter is clogged, these ‘dead zones’ allow algae to establish themselves.
Removing black algae from a pool requires both mechanical and chemical treatment. The process is somewhat involved, but following each step carefully will effectively clear the infestation.
Tools and products you’ll need include:
Don’t forget protective gear like gloves and goggles, as you’ll be handling concentrated chlorine.
First, scrub every visible black spot vigorously. Use a stiff pool brush to break through the algae’s slimy coating. Focus on all walls, steps, and floors, especially corners or behind ladders, where black algae hide.
For concrete or plaster pools, you can use a pumice stone or a crushed chlorine tablet on a pole to scrape off the algae heads. The goal is to remove as much of the algae mass as possible and expose the roots.
Next, vacuum or rinse the loosened algae out of the pool. If possible, set your vacuum to waste so the debris doesn’t go through your filter. This removes dead algae from the water. If your filter is even slightly clogged, backwash it now and clean it thoroughly. Black algae can become lodged in the filter media.
For the remaining spots, apply chlorine directly to the algae. A common method is to break a chlorine tablet in half and rub the broken edge onto the spot. The high chlorine concentration will penetrate the algae’s roots.
Alternatively, you can place a half-tablet in a floating holder and position it near the algae. Always do this carefully to avoid spilling too much in one place.
After scrubbing, shock the pool with a very high chlorine dose. Many pool professionals suggest tripling or even quadrupling the normal shock amount. For example, use three to four pounds of calcium hypochlorite shock per 10,000 gallons.
Pour the shock around the pool evenly, or sprinkle it directly on stubborn areas and scrub it in. It’s best to shock at dusk or night. Chlorine lasts longer when the sun isn’t burning it off. Let the pump run for 24 hours to circulate the chlorine.
After shocking, the water may turn cloudy. That’s normal. Over the next 24 hours, brush the pool again several times to remove any newly dead algae. You must not skip brushing even after shocking. Brush-wash-vacuum cycles might need to be done multiple times over a few days. Each time, any leftover algae should loosen for easier removal.
Once you have finished the second round of shock and brushing, clean your filter one final time. Backwash the sand filters or rinse the cartridges. If you used metal-based algaecide earlier, you may have more debris in the filter. Cleaning now ensures no live algae remain.
Some experts also recommend using a specialized black algae algaecide after the above steps. These usually contain copper or silver compounds. A “kill dose” of copper algaecide can help eradicate any remaining algae.
When you add a high dose of chlorine to shock your pool, it may shift the pool’s pH level. You’ll need to test the water and restore it to ideal ranges — pH 7.2–7.6, chlorine 1–3 ppm — so your pool is safe to swim in.
Black algae in pools are not highly toxic, but they create a slimy environment that allows harmful bacteria, such as E. coli, to thrive.
While contact with black algae is unlikely to make a healthy person seriously ill, swallowing algae-infested water can cause nausea, stomach cramps, and, in rare cases, more severe issues due to cyanotoxins. Pets drinking pool water are also at risk.
Until the algae are removed and water chemistry is restored, avoid swimming. Once treated and balanced, the pool is safe to use.
Preventing black algae requires consistent maintenance:
For a long-term solution to black algae in your pool, especially if recurring, you might consider resurfacing your pool. This defends against black algae by sealing the microscopic pits, cracks, and rough patches where spores love to lodge and multiply. A fresh layer of plaster, aggregate, or pebble finish creates a nonporous barrier that makes routine brushing far more effective.
For homeowners ready to tackle this upgrade, our top tips for pool resurfacing walk you through selecting the right finish. You can also tap into swimming pool financing to spread out the cost over time.
Copper- or silver-based algaecides designed for black algae work best. These products penetrate the protective coating and reach the roots. Apply a “kill” dose after scrubbing and shocking. Avoid standard green-algae algaecides, which are not strong enough.
Shock helps, but usually is not enough on its own. Black algae have a slimy layer and deep roots that resist normal chlorine levels. Use a high-dose shock of three to four times the normal amount at dusk, then brush thoroughly. Repeat as needed until all spots are gone.
Look for tiny black or blue-green spots with a rough, raised texture on walls, floors, or steps. These spots will not be easily brushed away. Even a vigorous scrub often just loosens them.
If they resist regular brushing and appear in corners or behind ladders, it’s likely black algae rather than mineral stains.
Act quickly and follow these steps:
Algae return when any roots or spores survive and favorable pool conditions persist. Common causes include lapses in sanitizer levels, infrequent brushing, poor circulation, or a dirty filter.
