The appearance of black algae in the pool is one of the most common problems in the maintenance of public or single-family pools.
They appear as black spots in the pool on the walls, corners, floor, steps. In most cases, they form as algae colonies in the o-ring of the Glass mosaic tiles and do not disappear on contact.
In this article, we explain you everything: why black algae in the pool, how to remove black algae from the pool and how to remove black spots in the pool.
Black pool algae is actually a type of cyanobacteria, a life form similar to algae and plants. Cyanobacteria are as old as life on Earth. In fact, it is believed that it was they who performed the first oxygenic photosynthesis 2.7 billion years ago and it is thanks to these small life forms that the Earth’s atmosphere contains oxygen.
Cyanobacteria are “extremophilic” organisms, which has allowed them to colonize all types of habitats and makes them highly adapted and difficult to eliminate.
In the case of the swimming pool, they develop in water that has not been well maintained, especially with low chlorine levels, uncontrolled pH and high percentages of nitrates and phosphates.
Cyanobacteria in swimming pools are usually Oscillatoria and Phormidium. This type of bacteria usually generate biofilm colonies in the pool with dark blue-green colors that, when combined with other pigments that these bacteria have, can cause the appearance to be black to the naked eye.
Both oscillatoria and phormidium can pose a serious health risk to swimmers, as both produce microcystin, a toxin of the cyanotoxin family.
Cyanotoxins are one of the most potent poisons in existence and both young children and adults with previous pathologies can be affected in case of direct contact. Symptoms of illnesses caused by exposure to cyanobacteria can range from stomach pain, headache, dizziness, vomiting, diarrhea and, in certain extreme cases, liver damage.
For that reason, removing black algae from the pool is not a purely aesthetic issue, but also a health risk for the bather, especially the most vulnerable. In the case of pets, the effect of cyanobacterial toxins is even more serious, as dogs and cats can become seriously ill within hours after drinking water containing cyanotoxins.
If you want more information on this topic, we recommend you read our guide on how to care for your pet in the pool environment.
Removing black algae from the pool is very laborious, as they often incorporate very resistant layers of biofilm, which reduce the effectiveness of black algae products in the pool. Removing the biofilm requires a lot of patience, as we will have to brush several times to allow the chlorine to penetrate well into the algae colony.
1. Clean the pool filter thoroughly.
If your pool basin has black algae colonies, your filter will certainly have them too. Before starting treatment, perform a good backwash of the sand filter. If you have a cartridge filter, clean the filter intensely. The filter will be our ally during the process, and we will need it to be as clean as possible.
2. Balances pH, alkalinity and calcium hardness to ideal parameters.
This way the disinfectant will be more effective. Use pool water test strips or a multiparameter photometer such as the scuba 3s that measures phosphates. The levels we will be looking for will be:
Remember that the correct order to balance the water is: first the alkalinity and then the pH, then the Stabiliser and finally the chlorine.
3. Intensively brush the walls and bottom with a curved wall brush to remove black stains in the pool.
If black spots remain, they should be brushed more intensely, respecting the resistance of the support. If your pool has a liner that allows it, you can use a wire brush to help you remove algae left on the walls.
Another alternative is to use a pool chlorine tablet directly on the algae colony, scraping it off until it is eliminated. For this purpose, always use appropriate equipment such as gloves and goggles. This type of solution cannot be used in liner or reinforced pvc liner, nor in fiber pools, as it would damage the pool liner irreparably.
If the algae are in places with poor accessibility such as the bottom or lower part of the walls, use a pole adapter to help you reach the farthest areas. Make sure that everything is brushed and that no debris remains. It is a very hard task that requires patience, but it is essential to remove black algae from the pool.
4. Add triple dose of cartridge pool flocculant to the pool skimmer to increase filtration efficiency.
Add a double dose of pool phosphate remover that will act as an “anti-black algae in the pool”. Both products for black algae in the pool will help to better filter and remove the “food” of black algae.
5. Perform shock chlorination at four times the manufacturer’s recommended dose for normal shock treatment.
To do this, remember to dissolve the black algae products in the pool in a bucket of water and spread it conveniently around the perimeter of the pool.
6. Run filtration for 24 hours.
This will allow the shock chlorine to be distributed throughout the pool and the algae, which we have put in suspension by brushing the walls, to be retained in the filter, which we have previously reinforced with the flocculant cartridge.
7. The next day, brush all the walls again with an intense brushing.
Black algae are very resistant and tend to re-deposit on walls and bottom. In addition, with the dose of chlorine that we have done, the last biofilm that has remained in the pool, will have been destroyed, and it will be the right time to remove the most resistant parts of these algae.
8. Do an intense washing and rinsing of the sand filter.
If your filter is a cartridge filter, clean it thoroughly and, if possible, perform a specific maintenance treatment by soaking it in cartridge filter cleaning solutions. In this way, all the bacteria and residues that we have collected in our filter, will be eliminated from the pool.
9. Add an additional dose of shock chlorine, brushing the walls and floor again.
Keep Pool filtration system purifying for another 24 hours.
10. After 24 hours, repeat the washing and rinsing of the filter.
It rebalances the alkalinity, pH and calcium hardness parameters. Check the phosphate level in water and remove any remaining phosphate with phosphate remover.
During the first week, watch closely for signs of black algae reappearance. Be patient, they are usually very resistant. If this is the case and new micro-colonies of algae appear, brush again and treat with shock chlorine, filtering for 24h.
Surely the black algae (cyanobacteria as we said), have colonized other parts of your pool that are not in sight, such as the filter bed or pipes. We recommend a thorough cleaning of the filter at the end of the season, using a specific sand cleaner for pool filters.
Preventing the appearance of black algae, and in general of any type of algae (pink, mustard, black, green…), is simple if a series of simple guidelines are followed:
Black algae are actually cyanobacteria. Because cyanobacteria produce a highly toxic poison, their presence in the pool is not only an aesthetic problem, but also a potential health problem for swimmers and pets.
Therefore, it is important for you to know how to both remove black algae from the pool and how to remove the black spots they cause. If you have doubts or have not been able to get rid of black algae, do not hesitate to contact us via WhatsApp.
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