Green Hair Algae In Fish Tanks

Causes, Treatment, and Prevention

Hair algae on live rock

Kristian Gravvold / Flickr / CC BY-2.0

Both freshwater and saltwater aquariums can become infested by green hair algae. As the name suggests, this type of algae grows into long strings that resemble hair. Although mainly an aesthetic issue, long strings of this type of algae can actually entangle fish and invertebrates in your tank, making it difficult for them to move or feed. The algae grows fast, as well, and can become very unsightly. Removal of green hair algae is not an easy process, but prevention is key to keeping it from coming back. 

What Is Green Hair Algae?

Algae are simple plant-like organisms belonging to the kingdom Protista. Like plants, algae can carry out photosynthesis, but lack the true roots, leaves, and stems typical of the more complex plants. There are many different species of green algae that can take on a hair-like appearance, but most green hair algae belongs to the genus Oedogonium. This common scourge of the fish tank forms long strings, giving it a hairy appearance. It is also known as string algae.

While unsightly, green hair algae is not toxic to fish or invertebrates. However, thick mats can cause fish and invertebrates to become entangled, keeping them from eating. For some varieties of fish with fewer scales, such as Doitsu koi, hairy algae can cut into the fish's skin and cause secondary infections. Extensive growth of algae can reduce light levels and oxygen in your fish tank, as well, which is potentially harmful to your fish and to aquarium plants.

As with all plants, green hair algae utilizes the carbon dioxide in the aquarium water and produces oxygen through the process of photosynthesis when light is present. However, when the lights go off, algae will start to respire and use the oxygen in the water. If you do not have additional oxygen being added to your aquarium water via an aerator or filter, the algae may out-compete your fish for oxygen, causing them to asphyxiate and die. This will also cause respiratory acidosis and drop the pH of the aquarium water. If the pH or alkalinity is too low, that can also kill your fish.

Symptoms of Green Hair Algae

There are many different types of hair algae that can grow in a freshwater or saltwater aquarium. The signs are fairly obvious and usually start as a small patch of green algae that quickly forms into longer "hairs."


  • Green growth that resembles hair on aquarium glass, substrate, and decorations

This type of algae forms by slowly linking up individual cells into longer chains, so you may see it get longer over time. If left alone, some species of hair algae can grow several inches long within days. The algae can grow on any structure within the aquarium, including the substrate at the tank's bottom, on the decorations and filter inside the tank, living or faux aquarium plants, and the inside of the tank glass.

Causes of Green Hair Algae

Unfortunately, no matter what kind of aquarium you have, fish + sunlight = algae. It only takes one cell of this simple plant to start growing a large population of stringy greenery. Even if you use a UV sterilizer on the water coming in with any new additions to your tank, you may have a cell or two of algae stuck to a fish, coral, or other invertebrate, which cannot be sterilized with UV light. Proper quarantine may help, but it is no guarantee.

Fish break down dietary protein to release ammonia in both saltwater and freshwater aquariums. Your biological filtration converts ammonia into nitrite and finally, nitrate. This nitrate is used as a nutrient by plants and algae. The more food for the algae, the more algae you can expect to have. The nitrate accumulates in the aquarium water unless it is removed by water changes. You can remove excess nitrate by performing more frequent water changes or increasing the volume of your water changes. It is highly recommended to use a gravel siphon weekly to remove excess food and debris from the substrate on the bottom of your tank.

In saltwater tanks, coral supplements such as phosphate can also provide fertilizer for algae. If you have a saltwater tank, but no corals, switch to a non-coral saltwater mix in order to keep your level of phosphate low. If you have corals and still have a problem with high phosphate, you may have to use a combination of coral and non-coral salt mixes to keep your corals happy and algae from growing out of control.

Lighting is another key component to algae growth. The more sunlight or artificial light on your tank, the more energy will be available for algae to grow and proliferate. Try to keep your aquarium out of the sunlight, as this will deter algae growth and keep a more consistent temperature. If your aquarium has artificial lighting, it is helpful to use an on/off timer on the light fixture to make sure you have the lights set for designated day (on) and night (off) periods. You may consider decreasing your day light periods and increasing your night periods to reduce algae growth, if that will not affect your tank inhabitants.


You can diagnose green hair algae in your fish tank simply by its distinctive appearance. Look for stringy, hair-like green growth that rapidly covers most interior surfaces of the fish tank.

Treatment of Green Hair Algae

Chemical additives and UV light may work well on more complex forms of algae, but not hair algae. The best way to remove hair algae from your aquarium is manual removal, combined with steps to keep your aquarium water well-balanced with the proper levels of carbon dioxide (CO2) and oxygen.

Use an algae scraper on the sides of your tank, making sure your equipment is specific to a glass or acrylic aquarium. For décor items, remove them from the tank during your regular cleanings, and scrub them with a soft brush, such as a designated toothbrush, under hot, chlorinated water.

For live plants, you can gently rub the algae off the leaves or perform a quick algaecide dip. For algae growing on substrate, the best method is to use a gravel siphon to turn the gravel away from the aquarium lighting. For algae growing on corals and invertebrate shells, you must be very gentle. Use a small airline suction device to carefully remove algae growth from these delicate creatures. Never scrub them, as this can cause injury.

How to Prevent Green Hair Algae

Prevention is the key to successfully battling algae in your tank. The following tips will keep your algae under control for both saltwater and freshwater aquariums.

  • Add fish to your aquarium that naturally dine on algae. In a saltwater tank, tangs, blennies, and rabbitfish are possibilities. In the freshwater tank, consider plecostomas, mollies, and Amano shrimp.
  • Keep a close eye on your nitrate and phosphate levels. Perform proper maintenance and make sure your salt mix is appropriate for your tank inhabitants (coral vs. non-coral). Regular water changes are crucial for keeping nitrate and phosphate from building up to unhealthy levels.
  • Use appropriate lighting for your tank inhabitants and keep your tank out of natural sunlight. If you use artificial lighting, reduce the amount of "daylight" hours slightly to hamper the growth of algae. Be diligent with your cleanings and remove hair algae as soon as you notice it in your tank.
Article Sources
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  1. Aquarium Water Quality: Total Alkalinity And Hardness. Florida Department Of Agriculture & Consumer Services