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What Causes Hair Algae? (In Freshwater & Saltwater Tanks)

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As a fish owner who has been around this hobby for years, I can’t remember how many times I have come across hair algae. And every time I did, I couldn’t help but wonder what makes this interesting species grow in fish tanks.

These are usually involved in the growth of hair algae:

  • The presence of nitrifying bacteria in cycled tanks.
  • Items that have been added to the tank and carry hair algae residues.
  • High concentrations of nitrate and phosphate.
  • Excessive light.
  • Low CO2 (carbon dioxide) concentrations.

In this article, I will discuss everything you need to know about the causes of hair algae in both freshwater and saltwater tanks. Next, I’ll share some great tips on how to control hair algae and prevent them from spreading.

The cotton-like form of hair algae.

What Causes Hair Algae?

Because hair algae are rarely beneficial to aquariums, you should learn to identify the sources that attract the organism so you can fight it. Common culprits include:

1. Hair Algae Naturally Grow In Cycled Tanks

Cycling allows aquatic creatures to safely inhabit the aquarium by nurturing Nitrosomonas and Nitrobacter, which process ammonia and nitrites, producing nitrates that are relatively harmless.[1]

The cycling process matters because fish create waste that generates ammonia when it rots.

You cannot avoid the toxin. Fortunately, cycling introduces bacteria that protect fish from ammonia by consuming the toxin.

Algae and the cycling process go hand in hand.

A paper from Utah State University (Donald B Porcella, John C, Stube, Frederick J. Post) studied nitrogen cycling in the northern arm of the Great Salt Lake back in the 1970s.[2]

The paper noted that algae and the bacteria in the lake had a close relationship because algae created organic matter that bacteria consumed. 

On the other hand, the bacteria produced ammonia that algae utilized in their growth

Don’t be surprised if hair algae appear in your tank during the cycling process. After all, cycling deliberately creates ammonia in an effort to rear beneficial bacteria.

Some sources have even identified algae growth as a sign of a fully cycled aquatic system.[3]

Keep in mind that beneficial bacteria turn ammonia into nitrates, a substance that encourages hair algae growth.

Therefore, if you can see algae and the nitrate concentration is high, your tank is probably ready to accommodate fish.

Nitrates produced by nitrifying bacteria are typically consumed by plants & algae.

2. You Accidently Added The Algae To Your Tank

Some hair algae species enter the tank via new additions, the most prominent being plants and decorations. 

Aquariums are tricky because people add and remove items all the time, which creates opportunities for dangerous organisms to spread.

Consider the habit some aquarists have of moving filter media from established aquariums to their cycling tanks to expedite the cycling process. They’ve also done this with gravel. 

And what about the aquariums they buy from other aquarists or the pots and vases they get from fish stores? Algae has plenty of opportunities to spread.

Consider this paper from the University of British Columbia’s Department of Zoology and Biodiversity Research Center (Shing Hei Zhan, Ting-Chun Kuo, Shao-Lun Liu, Lan-Wei Yeg, Tsai-Yin Hsieh, Shoichiro Suda).[4]

The survey blamed the aquarium trade for the spread of red macroalgae. Apparently, the algae were using aquatic plants and animals to infiltrate local ecosystems.

This other paper from the University of Wisconsin (Katherine McMahon) was looking at Nutrient Cycling Responses when it noted that owners of marine aquariums would often add live rocks covered with crustose coralline algae to their tanks.[5]

Live rocks are not alive, despite what the name suggests.[6] The term refers to the remains of dead corals. People take them out of the ocean, unaware of the algae the live rocks carry. 

The Victoria University of Wellington highlighted the threat tropical algae that rode into New Zealand on corals posed, proving that the dangers of algae are not limited to their activities in your home aquarium.[7]

If new hair algae species appear whenever you introduce plants and decorations to the tank, now you know why.

Ways to know if you accidentally added hair algae to your tank:

  • You have recently bought decorations or plants.
  • The hair algae appeared during the first two weeks of the purchase.
  • The algae first grew on the items added to the tank.
Hair algae could have been already attached to rocks and decorations that came from outside.

3. The Nitrate And Phosphate Are Too High

You don’t need an aquarium to get a first-hand glimpse at the relationship between nitrogenous waste and algae growth. 

Experts from the School of Earth Sciences (Stanford University) blamed significant algal blooms in the sea on agricultural runoff.[8]

They reached this conclusion after analyzing satellite images of the Gulf of California. 

This is not the only team that has connected agricultural waste, and especially fertilizers, to algal blooms. 

In an aquarium, your biggest concerns are nitrates and phosphates.

Shakouri A and Balouch G.M (Iranian Journal of Fisheries) proved as much when they investigated the effects of nitrates and phosphates (at different concentrations) on macroalgae.[9]

Unsurprisingly, they found that nitrates and phosphates encouraged algae growth. 

Additionally, a team from Scientific Reports (Journal) wanted to record the uptake rates of nitrogen and phosphorous (originating from terrestrial runoff in the aftermath of a storm) among the various species in a coral reef community.[10]

Again, unsurprisingly, the turf algae and macroalgae benefited greatly from the nitrates and phosphates. 

If you have high nitrate and phosphate levels in your aquarium, hair algae will grow at a rapid pace.

Common sources of nitrates and phosphates include tainted water sources, dead plants and animals, and rotting leftovers. 

Ways to know if hair algae bloom due to excessive nitrate and phosphate:

  • The nitrates are higher than 20 ppm.
  • The phosphate is higher than 0.2 mg/L.
  • Your tank is fully cycled.

4. Your Tank Is Exposed To Too Much Light

Fish tanks require balanced lighting. You can’t overwhelm them with artificial lighting. However, they don’t appreciate continuous darkness. 

They need an equal amount of daylight and darkness. Otherwise, hair algae will not only appear but grow out of control.

This is why aquariums positioned next to windows are constantly struggling with algae. Direct sunlight boosts algae growth. 

Ways to know if hair algae grew due to excessive light:

  • The edges of your plants turn brown or yellow.
  • The plants seem shriveled and rotten.
  • Besides algae, the plants in your tank grow quickly.
  • The water temperature keeps rising.
  • Your aquarium is placed next to a window.

5. There Isn’t Enough CO2 In The Aquarium

Hair algae respond positively to little or no carbon dioxide. Live plants are the opposite. They rely heavily on carbon dioxide to thrive. 

You can use a carbon dioxide checker to investigate the CO2 content in the tank. It comes with a liquid that turns blue when the aquarium doesn’t have enough CO2.

It is also worth noting that you can estimate the carbon dioxide levels by measuring the pH and KH in your tank:

Low CO2 levels cannot boost hair algae growth on their own. You need a combination of factors, including intense lighting and imbalanced nutrients.

Here are some signs of a CO2 deficiency in a fish tank:

  • The CO2 was measured below 15 mg/L.
  • Your tank lacks agitation.
  • Your aquarium plants keep dying or don’t grow properly.
  • The pH and ammonia keep rising.

How Do I Control Hair Algae In My Aquarium?

Controlling hair algae is far easier than removing the organism once it takes root in your aquarium. 

Fortunately, you don’t have to take any drastic steps to control hair algae once they grow. Consider the following:

1. Use Natural Algae Eaters

Some people hate adding algae eaters like Amano shrimp to an aquarium with fish because they risk overcrowding the tank. 

You can avoid this headache by filling your aquarium with algae eaters from the start. 

Look for Cherry Shrimp, Siamese Algae Eaters, Ramshorn Snails, and other species that actively seek out algae.

The creatures will attack the hair algae during its earliest stages before it can overrun the tank.

Some aquarists will starve their algae eaters on occasion to encourage them to eat the algae, especially if the hair algae are running amok and the creatures have chosen to ignore it.

All in all, you don’t have to worry about accommodating new algae eaters to fight a hair algae infestation when your aquarium already has algae eaters.

Useful info: If you wish to use algae eaters to control the hair algae situation in your tank, here is an article where I listed the best species for this job.

Ramshorn snails are excellent algae eaters and can be used to control hair algae growth.

2. Routine Maintenance & Water Changes

Maintain the aquarium to the best of your ability. Try to tick all the obvious boxes. 

That includes performing weekly water changes and buying a decent filter that can keep rotting organisms out of the tank.

A strict maintenance routine will keep the nitrates and phosphates at harmless levels. Hair algae are most common in neglected tanks with a nutrient imbalance. 

You don’t need water conditioners that remove nitrates and phosphates when your tank is clean.

Although, that shouldn’t stop you from buying water conditioners and special filter media that attack toxins.

3. Reduce The Lighting Duration & Intensity 

You should keep your aquarium away from natural light because it enhances algae growth. 

However, artificial lighting is necessary. The plants will consume oxygen and produce carbon dioxide in darkness, which is not good.

More importantly, your fish want the light. Rather than removing the lights, avoid excess lighting. 

Give the aquatic environment 8 hours of light per day. Use timers that shut the lights off after 8 hours, just in case you forget.

Use just enough light to see the fish. Keep the power below 1.5W per gallon. 

High-output lighting (Metal Halide Bulbs, LED Arrays, etc.) is beneficial when it allows plants to outcompete hair algae where nutrient uptake is concerned.[11]

A paper in ‘Advances in Difference Equations’ suggests that macroalgae show higher growth rates under the red wavelength.[12]

However, you shouldn’t overthink the lighting issue. The wavelength hair algae respond to doesn’t matter to you.

So long as the lights are not too bright, and you restrict the lighting in the tank to 8 hours a day, the hair algae won’t run amok.

It is crucial to limit the aquarium lighting to 8 hours a day.

Pro tip: If you suffer from hair algae and wish to get rid of them, here is an article where I explain how to do that properly, step-by-step.

Will Hair Algae Die Off?

Hair algae appear and grow because of an imbalance in nutrients. This is why the organism is so common in cycling tanks. 

Once you balance the nutrients and initiate an effective maintenance routine, the hair algae will disappear. But this doesn’t happen overnight. 

The algae can cause further imbalances if they die and you permit the organisms to rot without removing them.

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The occurrence of hair algae doesn’t necessarily mean that there is something wrong in your tank. It could merely indicate that your tank is cycled and inhabits beneficial bacteria.

However, hair algae could also flourish due to excessive light and nutrients, with nitrate and phosphate in particular.

You can control this type of algae by limiting what makes it flourish. That can easily be done with routine water changes and proper aquarium maintenance.