Skip to content

How to Get Rid of Blackspot Algae

Blackspot algae is one of the most stubborn types of algae you will encounter as a pool owner. It is notoriously difficult to get rid of these. They have deep roots and when you notice a black spot in your pool, you might be too late.

But don’t worry, you will not have to build another pool to get rid of these. Plus, it won’t be effective, their spores can survive in really harsh conditions. It is important to get informed about all the options. We will help you get rid of Blackspot alga and believe us, our tips are spot on!

What is Black Spot Algae?

Blackspot Algae is a type of algae that can occur in concrete pools. As the name suggests, blackspot algae usually look like a spread of black, dark green or even occasionally blue spots embedded in the pool's surface.

It's different from the much more common green and mustard algae, it grows slowly but is also harder to treat. Blackspot algae tend to grow in the parts of the pool that are hard to reach.

Why is it so hard to remove these algae? It has too many layers of protection from chlorine. If you remove only the dark part of the algae, it will be back in just a couple of days. Blackspot Algae grows slowly and patiently and once they claim your pool, it will be hard to get rid of it.

Their spores can survive on the pool surface even when the pool is empty. When you fill your pool with water again, guess who is coming back in just a few days?

What Causes Blackspot Algae Growth In Your Pool?

The causes of Blackspot Algae growth in your pool can be various. It is most often caused by high phosphate levels or by poor chlorination. Blackspot Algae are feeding on phosphates which stimulates their growth. Yet, they can survive even in waters with low phosphate levels. So the first step in prevention is adequate chlorination.

Other factors that can cause Blackspot Algae growth include:

  • High pH levels
  • Worn pool interiors
  • Foliage around your swimming pool

How Do I Prevent Black Spot?

Working on preventing black spots in your pool is maybe the best way to save your time and your nerves.

If you own a chlorinated pool, you should aim to keep the free chlorine levels above 1.5 parts per million. This will stop the growth of any type of algae.

Chlorination is the first step, but you need to make sure that all the chlorine is distributed evenly in your swimming pool. Check on the water circulation. During the season, you should run your pump between 8 and 12 hours daily. Do this while keeping the pH levels between 7.4 - 7.6. Alkalinity levels should be between 120 and 150 ppm all the time.

Vacuuming your pool and removing debris are also essential to prevent algae growth. Use the pool covers to prevent debris from falling into your pool. Clean all the additional equipment like pool toys, ladders etc.

Green Algae vs Blackspot

In order to help you understand all the differences between types of algae, we gathered this information for you.



What pools can get it?

Any type of swimming pool

Concrete/Concrete variant pools only

Ideal Growth Conditions

Low Chlorine, Hot, High Humidity

Same as green, but also thrives in high pH environments

Speed of Growth

Very quick if left untreated can turn into a major problem within a day or 2

Slower, generally takes longer to grow and spread

Difficulty of treatment

Relatively easy, especially if you act quickly

Relatively difficult, but exponentially worse if left to grow

How to Treat Black Spot Algae?

The steps outlined below work for the majority of blackspot algae blooms, however, if the coverage is extreme (nearly covering the entire pool) it's recommended that you contact a pool professional to drain and acid-wash the surfaces of the pool.

  • Vigorously brush all of the affected areas with a stainless steel pool brush (MUST be metal, nylon brushes will not do the job)
  • Turn the pump off for 2 hours to allow the water to settle
  • Apply treatment to the affected areas, depending on size
    • If light algae coverage then spread 1kg of LoChlor Blackspot Algaecide 900 on top of the affected areas
    • If medium algae coverage spread 2kg-4kg of strong granular chlorine (min 700g per kg) over the affected areas
    • If widespread algae coverage, spread 10kg of strong granular chlorine (min 700g per kg) over the affected areas
  • Leave the pump off for an additional 24-36 hours to allow the treatment to sit on top of the affected areas and really burn out the algae.
  • Vacuum the affected areas, if possible vacuum to waste.
  • Add an appropriate amount of algaecide to the water (view dosage rates of your specific product). A metal-free algaecide is recommended as it tends to be more effective at treating blackspot algae compared to metal-based ones
  • Leave the pump running for a further 24-72 hours before bringing a water sample into your local pool shop and balancing as directed
  • Continue to brush the affected areas of your pool twice a week with a stainless steel brush for a month.

Blackspot in a Fibreglass Pool

As mentioned above, blackspot algae can only form in pools made of concrete or a concrete variant (such as pebblecrete). However, if you own a fibreglass pool you may also get a series of marks across the bottom of your pool that resemble blackspot algae.

However, rest assured it is not blackspot algae or anything similar. Unfortunately, blackspots in a fibreglass pool are signaling that your pool needs resurfacing.

Black spots in a fibreglass pool are caused by the breakdown of the actual fibreglass behind the gel coating in your pool. When it breaks down, it creates tiny holes in the gel coat allowing pool water to flow in, causing these tiny black spots/bubbles in your fibreglass pool. This only ever happens in old fibreglass pools, or pools that weren’t fibre-glassed properly.

Either way, the only real solution is to get the pool resurfaced by fibreglass professionals. However, a cheap temporary solution can be found in LoChlor’s Spot Away, which can help temporarily remove or reduce any staining in your pool caused by this osmosis.

In Conclusion

Black Spot is a really annoying visitor in the swimming pool. It often goes unnoticed until it gets too late. As a pool owner, you always need to take extra steps to prevent a safe and healthy environment. Black spot is both an aesthetic and health concern as well since they can harbour some really nasty types of bacteria.


How Do You Get Rid of Black Spot Algae?

Getting rid of the black spot is not easy, but it is not impossible. Depending on the level of coverage, you will need to use the metal brush for affected areas. Using algaecide the right algaecide will help you stop further growth and solve the problem in the core. Regular maintenance, cleaning and keeping adequate chlorine levels should be helpful to prevent black spots.

If you are facing severe coverage with black spots, we highly recommend hiring a professional.

What Is Black Spot Algae?

Black spot is a highly resistant algae strain. It appears as black dots on your pool floors, walls and surfaces. They possess a very hard shell that protects the inner cells. They are often quarter sized with deep roots.

Is Black Algae Poisonous?

No, it is not poisonous. The only problem is that algae can attract some really dangerous types of bacteria such as E. Colli or Cyanobacteria.

What Happens If You Swim In Black Algae?

Swimming in pools infested with black spots is not a very pleasant experience. Algae itself won’t hurt you but it represents a fertile ground for various bacteria.

Should I Drain My Pool To Get Rid Of Black Algae?

You should not drain your pool or apply acid wash to get rid of the black spot. It won’t help with fighting them off since they will be back the other day. Only scrubbing and the right treatment with algaecide can help you achieve long-term results.

Prev Post
Next Post
Someone recently bought a
[time] ago, from [location]

Thanks for subscribing!

This email has been registered!

Shop the look

Choose Options

Edit Option
this is just a warning
Product Enquires
Shopping Cart
0 items

Before you leave...

Take 20% off your first order

20% off

Enter the code below at checkout to get 20% off your first order


Continue Shopping