The article below is the most comprehensive explanation of Black Algae causes and cures that I have ever read. The cure is written for people that have cement grout. The only cure for Black Algae that has attacked silicone rubber grout is as follows:
This is a time consuming process so it is important that you address the problem as soon as it is detected.
Swimming Pool Care for Black Algae Spots
Black algae is a problem for many swimming pool owners. When you wake up and find black algae spots on your pool surfaces disaster has descended on your pool. It is best to handle this problem as quickly as you see black algae spots and not wait until you have a major job of handling it. Get ready to roll up your sleeves and do some work. Black algae spots are resistant to chlorine and just shocking / chlorinating or dispensing an algaecide in your pool will not rid you of black algae. There is no easy way to get rid of black algae. It will take muscle power, correct chemicals and vacuuming to eliminate this problem.
The supplies you will need to handle black algae spots: a stiff pool brush, chlorine tablets and algaecide. These directions are not for a vinyl lined pool. Do not swim in your pool during the black algae spot treatment.
Each black algae spot needs to be brushed with your stiff brush. Shut off the pool pump so the next step will be more beneficial. Using a chlorine tablet spot treat the black algae spot (not on vinyl lined pools). Next step, pour concentrated all purpose algaecide on the algae spot. With pump off leave the dissolved algaecide on the black algae spot over night. Next day brush your dead algae spots and vacuum to waste. If there are still some black algae spots you should repeat the process. After vacuuming start filter pump for circulation and adjust pH. Keep your pool chlorine level adjusted to 7.2 - 7.6 to prevent reoccurrence. Keep your chlorine residual at 1.0-3.0ppm.
Black algae spots occur in a swimming pool when your water has low free chlorine level and/or the pool chemicals are unbalanced. Good circulation is a must! The time took to avoid black algae is well worth your time and energy.
Thompson Polymer Specialists
Fiberglass Swimming Pools, Inc.
519 So. Leona Ave. | Lecanto FL, 34461 | Telephone: 352-746-6115 | FAX: 352-746-3903