Black Algae: A Prevention and Cleanup Guide

Ryan Cuenca
August 7, 2019

*WARNING* The information supplied in this article is extremely powerful and is all you will need to eliminate black algae from your pool. In it, we disclose our secret weapon for eliminating a small outbreak quickly and cheaply. Please read it thoroughly prior to making any rush product purchases as you may find the techniques laid forth herein will save you a considerable amount of money and frustration.

Diagnosing Black Algae Properly

Before treating what you think is black algae in your pool, let's first try to establish whether it is truly present (If you are certain you have it, please skip to the next section).

First, do you have a vinyl or fiberglass pool finish?

If so, chances are you are looking at something other than black algae. You see, black algae needs a porous surface to grow and flourish, and these smooth pool finishes don't allow for this to happen easily. On the other hand, if you have a plaster, marcite, or pebble finish, it could very well be black algae.

Secondly, is it one spot or many spots?

If you answered one spot, chances are you are not looking at black algae. It tends to come in patches and many spots, and not in a single, isolated spot.

Thirdly, is it highly resistant to your efforts to brush it away?

A network of black spots that are resistant to brushing very well could be black algae (NOTE: Some pools have anti-slip particles on the stairs and other places... If you can feel little poky things on the soles of your feet, these are not black algae)

The ultimate test of whether you have black algae is to scratch some off with your fingernail and examine it against a smooth, light-colored surface. If you see a tinge of green in the sticky mass, it is almost certainly black algae. The problem with this method is that it may require diving into the deep end, so make sure to take some deep breaths and throw on a pair of goggles!

Eliminating Your Black Algae Problem

OK, so we've hopefully helped you establish whether you are dealing with black algae. Next, we'll talk about how to get rid of it. But first, a word of advice and a few prerequisites:

  1. There is no magic cure for black algae. As you will read below, it will take time and persistence to rid your pool of this invader. Do not waste your money on products with the words "Black Algae" in the name; simply dumping a product in your pool will not solve your problem.
  2. Balance your pool water. Balanced pool water acts as a ward against any algae infestation and will help it from returning once you get rid of it.
  3. A high-quality stainless steel brush. You'll need this to rip off the outer layer protecting the filthy black algae colonies so that your chlorine can have maximum impact. *IMPORTANT* Only use a stainless steel brush if you have a plaster, marcite, or pebble finish. Do not use it on vinyl or fiberglass or you may tear a hole in your lining.
  4. Chlorine. Whether in the form of tabs, liquid, or granular, the chlorine will be all you need to drive the black algae back to the abyss from which it came.
  5. You can get a free pool maintenance plan from Poolhop and decide whether you want your pool sanitization chemicals delivered on a monthly basis.

Brush, Brush, Brush

The very first step in addressing your black algae problem is to brush all affected areas vigorously. So, brush, brush, brush away!

This can feel futile as your progress will not be visible. For when you brush black algae, your effort will not produce that satisfying and billowing cloud of material that you see when brushing a normal algae bloom.

But, rest assured, you are stripping away the black algae's protective coating, prepping it for the dose of chlorine you are about to administer. Just like a swab of alcohol before the shot!

To Bomb or to Snipe?

And here we part ways with many other Internet authors on the subject of how to defeat black algae. Most articles talk about shocking your pool into oblivion. As much as I enjoy doing this, sometimes it's just not necessary. Depending on the size and scope of your problem, shocking your pool could be like bombing an entire city in pursuit of a lone warrior.

Small Outbreak

Here's our secret weapon for a relatively small outbreak:

  1. Equip yourself with one 3" chlorine tab and a pair of gloves.
  2. Enter the water and scrape and scrub the tablet over the areas where black algae is present, just like you would with a magic eraser. Don't underdo it, but don't overdo it either. You should be able to see little pieces of chlorine in the cracks where the algae is.
  3. If you are not able to dive down to the outbreak or do not wish to get wet, get your hands on one of these handy-dandy devices. This tool will clamp a 3" chlorine tablet to the end of your pool pole, enabling you to scrub hard-to-reach areas.
  4. Let the chlorine sit there for a couple of hours, and then come back and inspect. You should see a remarkable difference already. Come back the next day and repeat if necessary, making sure to start by brushing the area again.

Large Outbreak

If, on the other hand, your outbreak is encroaching on a large section of your pool, shocking it will be the solution. Call in the bombers!

After brushing the affected areas vigorously and thoroughly, administer an initial dose of algaecide. Wait a couple of hours and then add a triple dose (3 lbs of shock for every 10,000 gallons) of shock to your pool, preferably in the evening for maximum impact. Run your pump filter for the next 24 hours.

Return the next day and brush affected areas once more. Continue brushing every day, several times a day if you can afford to. You should see the algae start to disappear.

If it doesn't at first, keep chipping away at it. Add more shock if you need to. Keep brushing. Unless it's an alien substance immune to chlorine, it will go away with persistent effort.

A Final Word

Black algae is notoriously persistent. It has deep roots, so even when it's not visible to the naked eye, it may be lurking just underneath your pool's surface. Don't let up your guard. Continue brushing the effected areas, and maintain a higher than normal chlorine level for a week or so after it appears to be gone.

Preventing Black Algae

Fast forward several weeks...You are sitting in your pool furniture, cold drink in hand, soaking in the sun, enjoying the surroundings. Especially your crystal clear pool surface.

This puts you in a reflective mood and you ask "How could I have prevented black algae from the get-go?"

Here are some tips to ensure it stays away once you've worked so hard to get rid of it:

  • Follow proper pool maintenance guidelines, which includes regular water testing and balancing and weekly upkeep tasks.
  • If you swim in a natural body of water (i.e., a river, lake, or ocean) wash your bathing suits before going in the pool with them. Cross-contamination is a likely source of black algae.
  • If you have an older plaster or marcite finish, consider refinishing it. Oftentimes, black algae is a problem in older pools with a finish that has developed cracks and pits over time.

Thank you for stopping by!

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