Pools - Black Spot Algae Factsheet

Swimming Pools Black Spot Algae Fact sheet for operators

Introduction

Black spot algae is difficult to remove as it embeds in the pitted walls and floors of swimming pools and on grout between tiles. The visible black spot is only the top of the organism. 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 become established in your swimming pool. Black spot algae can be prevented by:

 Maintaining a free Chlorine level of 1.5 ppm or greater at all times.  The thorough distribution of chlorine throughout the swimming pool by adequate mixing and circulation of the pool water, especially in the corners and the deep end.  Removing leaves and debris by vacuuming at least once a week to reduce the availability of nutrients and keep chlorine at high enough levels.  Disinfecting the walls and floors of a drained pool with a 1 ppm solution of sodium hypochlorite.

Control

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

If a swimming pool has a lot of black spot algae and cannot be controlled by raising the chlorine dose or scrubbing, the following procedure is recommended:

 Drain the swimming pool until only a few centimetres of water remain.  Using a stiff bristle brush or a high pressure water jet, clean the entire 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 completely wet the walls and floor of the swimming pool.  Completely wet the entire surface 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 infested with black spot algae should be scrubbed vigorously with the hypochlorite solution.  Continue to wet the walls and floor with the hypochlorite solution for a further half hour before hosing off.  If the water on the swimming pool floor is dirty it should be heavily diluted with additional fresh water allowed to drain away. This highly chlorinated wash water must then be neutralised with sodium thiosulfate before discharge. It must not be allowed to enter a waterway or stormwater 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 ppm to 2 ppm.

For alternative pool treatments for black spot algae, contact your local swimming pool shop for products such as Black Spot Remover.

This fact sheet was initially developed by Redland Shire Council, and is recognised and adopted by participating Queensland Councils.

1 of 1