Black Algae in my 20,000 gallon fiberglass pool

by Christine Dimon
(Dallas, TX, USA)

Over a 2 week period of time, I have emptied two bottles of black algae killer which looks blue when it is poured in with the skimmer working. I have kept the skimmer going 24 hours a day.

I brush with a nylon brush daily but the algae will not come up. I have cleaned the filter once every two days. I cannot use a metal brush on a fiberglass pool without damaging the gelcoat. I just spend over $7,000 on having my pool resurfaced 14 months ago. Please help.

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Hi Christine. If you're using something like Leslie's Black Algae Killer, what you're doing is adding a ton of copper into the pool. The label on that particular product says it eliminates and prevents black algae. Nothing could be further from the truth. Once a pool has black algae, it has it. Outbreaks can be infrequent, but they can happen. BA is called the herpes of pools for a reason.

Next, BA is not something that fiberglass pools get due to the fact of their smooth surface. BA has tentacle like features and burrow into the surface. Plaster pools are known for this, but not really fiberglass.

Without doing a couple of tests it's difficult to determine what exactly your issue is. What you can do is to place a chlorine tab on the affected area for a couple of minutes. If it lightens up it's algae and can be treated with high and maintained chlorine levels. If the area doesn't lighten up you can get some ascorbic acid which is crushed up Vitamin C. Put 1/2 lb. into a sock and place it on the affected area. If it lightens up you will need to do an ascorbic acid treatment. This is a metal remover.

Gel coats and fiberglass pools are wonderful, however, they don't like repeated and maintained high free chlorine levels. Doing this can cause chemical staining which cannot easily be removed. It is possible you might have chemically stained the pool, but again, that's difficult to determine simply by a quick question and answer.

A complete list of chemical readings would also help.

Robert