Causes of Green Hair Algae in a Saltwater Aquarium

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Few things are worse for your saltwater aquarium than a green hair algae invasion. At least aesthetically speaking.

It’s no wonder that green hair algae have become a pest to manage in saltwater tanks, especially due to their hardiness and ease of spread.

The algae’s adaptability is another notable issue here. These algae can thrive in any ecosystem, saltwater, and freshwater, and are notoriously difficult to combat.

They will return the moment you let your guard down and require some serious management to keep away. So, let’s get into that.

Causes of Green Hair Algae in Reef Tanks

While there are several causes for green hair algae to consider, we will look into the 3 of the most widespread ones that relate to reef tanks especially.

These include:

  • Excessive light – Light is the number one culprit here. Algae thrive in bright light, so they will quickly overrun a brightly-lit ecosystem. Light is naturally necessary for plants, corals, and fish, but you should always manage the intensity properly. Otherwise, your tank will be overrun with algae, and no amount of prevention strategy will work in such a setup.
  • Normal nitrate production – Nitrates are naturally produced in the tank by fish activity and normal organic matter decaying in the water. In a cycled ecosystem, ammonia and nitrites are broken down by denitrifying bacteria and turned into nitrates which algae use as sustenance. There’s nothing you can do to prevent the formation of nitrates, but you can minimize their impact on the environment. Plants consume nitrates, so have more live plants in your tank. Regular water changes are also necessary to dilute nitrates and starve algae as a result.
  • Coral supplements – Coral supplements contain phosphates which are also sustenance for algae. Combine these with plenty of lighting and the natural production of nitrates, and your algae will thrive.

Needless to say, algae are more prevalent in poorly maintained tanks with a lot of dead matter lying around and spiking nitrate levels.

If your reefs keep getting overrun by algae, you need to consider some serious algae eradication strategies, and that’s what we will be discussing today.

Is Green Hair Algae Bad for Fish?

Yes, green hair algae are bad for fish and plants alike. They’re not poisonous and won’t alter the water chemistry in any way, at least not at first.

But, as they grow, they become a hazard for several reasons:

  • The risk of entanglement – Small algae are harmless, but they can become deadly once they grow. The long green hairs can entangle fish by accident and actually drown them. It may sound weird, but fish need to swim to breathe properly. If they stand still, water won’t circulate through their gills properly, leading to oxygen deprivation and suffocation. It’s worth noting that algae don’t entangle fish on purpose but on accident; this only happens when the algae have grown quite large.
  • The risk of cuts – This is rarer, but it can happen. The longer algae filaments can cut scaleless fish, making them prone to secondary infections with a potentially deadly outcome. The same risk exists with long-finned fish for the same reasons.
  • Oxygen deprivation and CO2 poisoning – Algae are basically plant-like organisms that operate based on photosynthesis. They use environmental light to produce oxygen during the day, which aids in their normal physiological processes. The problem is that, just like plants, algae consume oxygen to produce CO2 in low-light conditions (at night.) In an environment overrun by algae, this equates to a high risk of CO2 poisoning and asphyxiation due to low oxygen levels. Such a scenario would turn algae into deadly organisms.
  • Light deprivation – This is a matter of serious concern in ecosystems overrun by algae. That’s because algae spread everywhere, on every surface they can find, including plants. As a result, plants will no longer receive the light they need and start dying. The additional dead plant matter will then produce more ammonia, resulting in more nitrates, further feeding the ever-growing algae population. And the deadly cycle keeps going.

As you can see, algae invasions are potentially deadly for your aquarium life, even if algae themselves are not toxic.

It’s their sheer presence and secondary effects that do all the harm. So, you need some viable solutions fast.

Getting Rid of Green Hair Algae in Reef Tanks

Now that you’ve identified the problem, you need a solid plan of attack to contain and eliminate the pesky green hair algae.

Here are the 3 fundamental strategies to employ:

– Reduce Nutrients

Adopt a thorough tank maintenance routine to prevent the excessive accumulation of nitrates.

This is one of the most reliable algae prevention routines to have in place, especially since it will also benefit your fish in the process.

You can decrease nitrates and phosphates by frequently changing water, adding more live plants to the environment, and eliminating dead matter frequently.

Also, monitor water phosphates and nitrates regularly to make sure they remain within acceptable parameters.

– Natural Algae Eaters

A variety of fish consume algae as part of their daily diet at various levels of effectiveness.

Some of the best to mention include tangs and surgeonfish, blennies, rabbitfish, angelfish, gobies, spotted scat, etc. You also have a range of invertebrates to consider, like shrimps, snails, and crabs.

Make sure that all species share the same environmental requirements if you house several of them.

Most importantly, make sure that they won’t eat each other or compete over too little algae, which can lead to tensions and fights.

– Reduce Lighting

Excessive lighting is the number one green algae predictor. I recommend adjusting the light levels according to your ecosystem’s requirements.

Light is necessary for plants, but not too extreme intensities, as different plant species come with different light requirements.

Fish need even less lighting. Most people use bright lights to increase the tank’s visual aesthetic, not because the fish need them.

Moderation is key in this sense. If you have corals, you don’t need more than 8 hours of light per day.

– Removing Nitrates & Phosphates

When it comes to managing phosphates and nitrates effectively, the overall maintenance routine, combined with the water change schedule, is just part of the solution.

If the situation is more severe than you might have expected, rely on phosphate removal products from the Seachem category.

These solutions are great for both marine and freshwater ecosystems, act fast, and have no chemical fallouts.

This makes them safe for your marine life while keeping algae at bay.


Green hair algae are notoriously difficult to combat in full adult form. You can circumvent this problem by tackling them in their early development phases, and today’s article pointed out the most effective methods.

I recommend adopting several prevention techniques for the best results.

Keep lights low-to-moderate for up to 8 hours per day, have several algae eaters lurking in the ecosystem, and remove algae manually if they spread too much.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.

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