Black spot algae is difficult to remove as it grows in the pitted walls and floors of swimming pools and on grout between tiles. The black spot is only the top of the algae. This part can be removed, but it will generally regrow within a few days.

Algal spores often take root on swimming pool walls and floors when the swimming pool is drained of water and grow when the swimming pool is filled with water.

Prevention

It is easier to prevent black spot algae than to control it once it has begun to grow in your swimming pool. Black spot algae can be prevented by:

• maintaining a free chlorine level of 1.5 mg/L or greater at all times
• adequate mixing and circulation of pool water so chlorine spreads through the pool, especially in the corners and the deep end
• removing leaves and debris by vacuuming at least once a week and keep chlorine at high enough levels
• disinfecting the walls and floors of a drained pool with a 1 mg/L solution of sodium hypochlorite.

Control

Small groups of black spot algae in an empty swimming pool can be cleaned by putting powdered chlorine directly onto the area, leaving it overnight and scrubbing it with a stainless steel brush the following day. Make sure you read the product instructions for personal and environmental safety. Do not use wire brushes on vinyl lined swimming pools.

If a swimming pool has a lot of black spot algae that cannot be cleaned by raising the chlorine dose or scrubbing, the following steps are recommended:

• drain the swimming pool until only a few centimetres of water remain
• use a stiff bristle brush or a high-pressure water jet, clean the surface of the swimming pool removing as much algae as possible. Be careful not to lift the paint
• prepare a 10:1 sodium hypochlorite solution, enough to cover and wet the walls and floor of the swimming pool
• completely wet the floor and walls of the swimming pool (protective goggles, gloves, rubber boots and overalls should be worn). Start at the shallow end and work towards the deep end
• all surfaces with black spot algae should be scrubbed with the hypochlorite solution
• continue to wet the walls and floor with the hypochlorite solution for half an hour before hosing off
• if the water on the swimming pool floor is dirty it should be heavily mixed with additional fresh water. This highly chlorinated wash water must then be mixed with sodium thiosulfate before discharge. The chlorinated water must not be allowed to enter a waterway or stormwater system as it will kill aquatic organisms. Note: 248 grams of sodium thiosulfate will neutralise 280 grams of chlorine
• refill the swimming pool with water and ensure the free chlorine residual is at least 1.5 to 2 mg/L.

For other black spot algae pool treatments, contact your local swimming pool shop for black spot algae removing products.