Black Algae in Swimming Pool

What causes black algae in swimming pools? If you’ve ever had to get rid your pool of algae, you know how frustrating the removal process can be. Black Algae is particularly stubborn (due to its protective coatings), and though while not harmful, is quite unsightly.

Algas grow when several things have gone wrong in your pool in combination. Therefore, if you want to eliminate the root problem, there are a few things you should check.

Here are the few most common causes of the bloom and what you should check  so you can ease that nagging question:

What causes black algae in swimming pools?


The problem could be stemming from something as simple as uncleanliness, so you’ll want to sanitize your pool first to eliminate the obvious suspect. Give your pool a deep clean with an additional step to target the unwanted growth.

First, remove  any foreign contaminants could be the source of the bacteria, or housing more of it (such as pool toys or leaves.) Be sure to sanitize any pool toys and equipment before putting it back in the water.

The special step in pool maintenance only ever applicable to remove black algae is that before continuing the wash, first scrub all the black stains by hand using pumice, afterward vacuuming up the shreds produced.

Run your automatic cleaners, sanitize all pool cleaning equipment afterward as normal, and consider shocking your pool two or three times as much as you did before. Brush hard and repeatedly, making sure you’re using one that’s appropriate for the task.

You may also want to backwash your pool. While the exact methods for this vary based on what kind of filter you have, in general, you’re going to just switch the handle on your system to the backwash position.

You should be able to see the water traveling through the backwash hose to be sure that it is running, so allow it to do so until the water runs clear.

Filter not working properly

Black algae like to grow in water that is not filtered properly. Spores are often dragged in on outside contaminants such as swimsuits, pool toys and floaties, or even just carried through wind and rain.

To prevent these tagalongs from harming your pool (or just turning into a big eyesore) be sure to clean out your filter to remove any leaves or trash that may be clogging it, preventing it from functioning properly.

After you’ve cleared it of any outside materials, be sure to clean the filter. Remember to wash the cartridge too, if your system includes one (many would recommend a chemical clean) so that any harmful spores are removed before they can grow or spread enough to do much damage.

Be sure to clean your filter weekly to continually protect yourself against black algae.

A high pH level


A high pH level is a very common cause of this unpleasant growth. Be sure to check on your pool’s chemical levels (as you should already do once or twice every week).

Check if to be sure pH levels are between 7.2 and 7.8, your alkalinity is between 80 and 120 parts per million, and your chlorine levels between a 1.0 and 3.0. If any of these factors differ, that could be why you have an infestation.

Next, check your pool’s circulation. One of the best ways to prevent algae growth is to keep your water moving; it prevents the spores from latching onto any surfaces, and helps distribute your cleansers and chemicals properly.

Be sure to examine your pump, as its functionality can very much affect how vulnerable your pool is to alga. It makes sure all of the water passes through your filter, so if your pump isn’t working, your filter might as well be broken too.

Ensure that is prepared to run for an appropriate length of time given the individual pool setup you have (typically 1 hour for every 10 degrees.)

Maintaining your pool infrequently

If it’s been a long, ongoing battle between you and black algae, it may be time to consider an algaecide. Some only use it when absolutely necessary, while others have made it a part of their weekly pool care routine.

Based on the severity of the situation more frequent treatment may be appropriate.

You may also want to consider that black algae won’t grow in certain kinds of pools. If you really can’t rid yourself of the problem, maybe your next remodel will give you the opportunity to start fresh and have it out of your life for good.

If your pool is completely overrun with black algae, and replacement isn’t an option, be sure to sanitize your pool and use plenty of algaecides.

Prevent black algae from coming back

After you’ve checked up on all the usual suspects, do another deep clean of your whole pool for good measure. Remember, with black algae, sometimes patience is key it may not go away right away. Be aggressive in your cleaning and check the usual suspicious factors regularly:

  • Cleanliness

  • Filtration

  • Chemicals

  • Circulation

  • If all else fail, try an Algaecide!

Be persistent until you see change, and then repeat the process again. It’s difficult, but the clean, pristine pool view will be worth it in the end!

We hope that this article about “What causes black algae in swimming pools?” was able to help you. If you have any questions or comments feel free to comment or send me a message!