Black Algae
This usually appears tiny BLACK SPOTS on the pool walls, steps,
corners, or around wall fittings.
This type of algae is the hardest to remove, since it is very
resistant to chlorine. Low chlorine and is usually the main
cause of this algae. Salt water pools which have a normally
low chlorine level, 0.2 to 0.5 ppm are the most prone to this type of algae. 
(click
to enlarge)
Solution:
1. Balance pool water: Try to get the PH (acid) level between 7.4 and 7.8. You may need to add muriatic acid if the PH is high and soda ash if it is low.
**Note** if you are having trouble getting the PH into the range above, test your tap water. I have found some pools on well water may have a very high or low PH depending on the area. If this is the case, proceed to the next step and don't worry about the PH level.
2. brush algae : This is the most important part in successfully removing Black algae. Feel the spots, they will feel slightly raised. The outer layer of the algae spot is actually dead creating a protective shell around the algae. Using a metal pool brush, brush each spot to completely remove the raised portion of the spot. There will still be a light green dot where the spot was. Don't panic, the next steps will take care of that. Remember it is important to scrub off all spots, so not to leave any live algae in the pool.
3. Vacuum pool: Vacuum up all loose algae that is now on the pool floor.
3. Backwash filter (if filter is cartridge type, clean elements) : Often the organic matter that is causing the algae is in the filter, cleaning it now will improve chances of completely removing algae.
4. Algaecide : Add algaecide. I have found
Swimtrine
Black
to be the most effective. Follow instructions for "visible
algae" as stated on the bottle. This will help kill and
prevent the algae from returning. I have found that it is not
necessary to continue on the maintenance dosage once the algae is
gone, since this product is high in copper and may stain the pool.
5. Shock pool : Add powder type of Chlorine also called "shock". With the pool off sprinkle shock so that it lands on top of the spots. Take your time, this is to bleach out the green dots left behind after brushing. Spots along the walls can be treated by letting the shock run down the walls passing over the spots. It may take 5 or 6 lsb. of shock to cover all affected areas. I recommend Calcium Hypochlorite(Cal-Hypo) because it has a high percentage of chlorine and it is the least expensive. It is essential to increase sanitizer levels to eliminate organic matter that algae feeds on.
**note** Salt pools are chlorine pools! adding chlorine to these pools is safe and will not have negative affects on the system.
6. Chlorine Tablets: Addition of Chlorine tablets will help to keep the chlorine level higher. Tablets are slow release chlorine whereas shock is instant release. I use a chlorine float, the benefit is that even when the pool is off chlorine is still being added to the water.
8. One last thought. For best results brush and shock pool weekly, remove bugs, grass clippings and leaves daily, keep the Chlorine floater full, run the pool 6 to 8 hours daily and backwash filter monthly. Have a great algae free summer!