Green Hair Algae in Planted Tank – Causes and Solutions

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Nobody wants algae in their tank, but everybody will get them. Green hair algae are common in the aquarium world and will begin to spread as soon as the optimal conditions have been reached. The problem is that the bar is extremely low.

It doesn’t take much for algae to grow and bloom since these are hardy and extremely adaptable organisms. But what exactly qualifies under the definition of ‘optimal conditions for algae?’

Today, we will discuss the common causes, solutions, and preventing measures regarding algae growth in home aquariums.

6 Causes of Green Hair Algae in Planted Tank

As a novice aquarist, you should always be prepared to face an algae invasion at one point or another. Green hair algae are pretty much omnipresent and will grow in any aquatic setup, provided the ideal conditions arise. Your job is to make it more difficult for the algae to bloom and invade the entire space since that’s harmful to both fish and plants.

So, let’s look into the 6 most common causes of algae bloom in home tanks:

– Excess Light

While direct sunlight is beneficial to fish and plants, it will also promote algae growth. You should always keep the tank in a well-aerated room, safe from direct sunlight. You can easily replace natural sunlight with a standard aquarium LED-based lightbulb. However, there are two core facts to remember here:

  • Light intensity – The more intense the aquarium light is, the better and faster the algae will spread and grow. Make sure that the light intensity falls within the manageable parameters. What those are depends on your plants’ requirements. Fish don’t have any use for overly-intense lighting. Most of them will thrive in low-to-medium light conditions. The lighting system is more important for plants since it helps in photosynthesis and nutrient absorption. It’s worth noting that different plants require different light intensities. So, always understand what your plants need and cut down on the light intensity whenever possible.
  • Light duration – Don’t keep the lights on for more than 8-10 hours per day at most. Not only will this help plants to function properly, but will also keep the algae population in check. The photosynthesis process reverses when light intensity goes down, causing plants to use oxygen to produce CO2, whereas the opposite is true during daytime. Keeping the lights on for too long will cause plants to produce excess oxygen, and algae love excess oxygen.

So, when talking about excess light, we’re actually talking about 2 different points: light intensity and light duration. Tweak these values accordingly, and your algae-control strategy will improve dramatically.

– High Nitrate Level

Algae consume nitrates and phosphates, the same as plants. These chemicals are byproducts of dead matter and fish waste accumulating in the tank. Generally, nitrates don’t impact water quality too much, compared to nitrites and ammonia. They even serve as food for plants, improving their growth rate, coloring, and overall health over time.

The problem is that algae and plants compete over similar nutrients. And nitrates and phosphates are always present in the tank, usually in higher quantities than in the wild. That’s because the tank is a closed system with no real chemical circulation. A well-cycled tank will produce nitrates constantly via bacterial activity, synthesizing ammonia and nitrites and producing nitrates regularly.

The effect grows exponentially based on the number of fish available, how much poop they produce, how much food residues there are, and the tank’s size, among others. Nitrates are also an active component in plant fertilizers, both root tabs and liquid versions for floating plants.

With so many sources of nitrates, you should expect to experience elevated levels at some point. Plants will consume some of them, but they cannot consume everything. The rest will go on to feed the growing algae population, and the more there are, the faster the algae will grow.

So, always monitor the nitrate levels in your tank and embrace a steady tank maintenance routine to lower them. Removing fish poop, food residues, and dead matter from the tank occasionally is a good strategy in this sense. As is adopting regular water changes designed to freshen up the environment and dilute nitrates and other dangerous chemical components.

– Lack of Water Flow

Algae hate slow-moving waters almost as much as they hate fast-moving ones. Stagnant waters represent the ideal algae environment because they allow the organisms to latch onto various surfaces and spread throughout the habitat. Fortunately, few aquariums deal with stagnant water thanks to the availability of filtration systems.

These will create water currents powerful enough to control algae growth and spread. But be careful about it! Some fish species despise water currents, causing them to exhibit signs of stress and even health issues over time.

Strong water currents may also affect the more sensitive plants in the tank, causing broken leaves, stems, or even unearthing. Make sure you adapt the filter’s power to your tank life while still ensuring a healthy and steady flow.

– Lack of CO2

Algae despise CO2 and will thrive when there isn’t enough of it in the tank water. This is bad news because CO2 is omnipresent in nature but extremely scarce in a closed aquatic setup. CO2 is the natural result of fish and bacteria respiration in the water. These life forms consume oxygen and eliminate CO2 as part of their normal physiological functioning.

Plants also produce CO2 during the night via a similar activity, consuming oxygen in the process. Plants consume CO2 during the day, and they need a lot of it. Some plants require more CO2 than others, especially if they’re bigger. Unfortunately, there sometimes isn’t enough CO2 for plants to absorb properly, which can lead them to experience nutritional deficiencies.

Algae bloom is another direct effect of low CO2 levels. This immediately gets us to the obvious solution to the problem – CO2 injections. Injecting additional CO2 into the environment will boost your plants’ health and functioning and suffocate the existent algae population.

Just make sure that your plants can cope with the addition of CO2 or that they even require it. Some plant species don’t need a CO2 boost, at which point adding CO2 will only hurt the environment. Also, fish and other aquatic animals don’t require CO2. This means that high enough quantities of CO2 will actually work against them, causing difficulties breathing and even suffocation in some cases.

– Dirty Filter

A dirty filter is almost always synonymous with a clogged filter. All filtration systems require some maintenance to keep them running at maximum efficiency. Filters accumulate a lot of dirt, fish waste, food residues, chemicals, and even substrate particles small enough to get in.

These will eventually form a thick muck over time, causing the filtration system to jam. If that doesn’t happen, the filter will, at the very least, lose its efficiency considerably. This means lower or no water flow and a more accelerated accumulation of nitrates. All these are known to support algae development.

To prevent this problem, consider adopting a set-in-stone maintenance routine to keep your filter clean and running at top efficiency. When it comes to cleaning the filtration system properly, consider the following:

  • Avoid tap water – Only use aquarium water when cleaning the filter. Tap water contains chlorine which will inevitably contaminate the filter and poison the water. Fish are extra sensitive to chlorine since this is literally toxic for them. Even if the chlorine present in the tap water doesn’t carry over to the tank, it will inevitably kill off much of the filter’s beneficial biofilm. As a result, your population of beneficial bacteria will drop dramatically, causing ammonia spikes in the tank.
  • Avoid chemicals – Only clean your filter with plain aquarium water – no chemicals of any sort. The idea I mentioned at the previous point stands here as well. Using any type of chemicals will sterilize the filter and eliminate all bacterial forms inhabiting it. Including the beneficial ones.
  • Avoid overcleaning – Only clean the filtration system superficially. In other words, eliminate visible dirt deposits and much that may clog the system and stop there. The goal is to spare the filter’s biofilm from an over-energetic scrubbing.

– External Contamination

Algae will often come with the tank via contaminated pieces, be them plants, decorations, or even used tank equipment. To prevent this problem, always clean, sterilize, and, depending on the case, quarantine anything going into your tank. I’ve written a more extensive article on the issue providing useful information in this sense.

Does Flourish Excel Remove Green Hair Algae?

Flourish Excel is a highly effective algaecide thanks to its active component glutaraldehyde. This is a chemical with vast uses in medicine and herbicides, which is to say that it will also kill plants in high-enough concentrations. Fortunately, aquarium plants are more resilient than algae in this sense.

Always read the product’s label before using it to ensure you’re not also killing your plants in the process.

Is Green Hair Algae Bad for Fish?

Green hair algae aren’t typically dangerous to fish. They don’t change the water’s chemistry and don’t produce any chemicals that could interact negatively with your fish’s systems. That being said, green hair algae can affect your fish and plants in the long run, as such:

  • Entangling fish – Mature green hair algae will grow quite large, carrying the risk of entangling fish and other tank inhabitants. This usually means death because fish need to swim to be able to carry oxygen-filled water through the gills and breathe. They will drown when forced to stand still.
  • Cutting fish – This may sound like a weird one, but it’s true. Some fish species have fewer or no scales, making them more prone to injury. Even if the fish can release itself after being entangled by the algae, the event may cause the algae to cut into its skin. Even slight and superficial cuts can mean death in a bacteria-filled environment, as they can lead to secondary infections fast.
  • Killing plants – Overgrown algae will cause significant damage to plants over time. That’s because algae can latch onto any surface and will even cover the plants’ stems and leaves. This will restrict the plant’s access to sunlight, causing severe nutrient deficiency and preventing adequate photosynthesis. This is generally a death sentence.

So, while green hair algae are harmless when young, they can quickly become killers when mature.

What Fish Eat Green Hair Algae?

The Siamese flying fox, bristlenose pleco, mollies, ameca splendens, otocinclus, etc., will all consume green hair algae whenever possible. You can even use crustaceans for the same purpose, as ghost shrimps and Gammarus crustaceans will happily consume them as well.

These creatures are ideal as algae control agents since they prevent the organisms from maturing and taking over the tank. If they don’t seem to be able to control the algae’s spread, consider other algae removal methods as well. These include manual removal, CO2 injections, adding more live plants, etc.

Feel free to review some of my other articles on the topic for a more in-depth perspective.


Green hair algae aren’t necessarily bad. They occupy quite a stable role in the food chain, serving as nutritious meals for various aquatic life forms. The problem is that, once algae bloom, problems begin to arise.

You’re now armed with the knowledge necessary to control and prevent algae bloom moving forward.

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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