Filamentous Algae – How to Get Rid of It in Your Aquarium?

Filamentous Algae

Filamentous Algae

Filamentous algae are sometimes called pond scum, and they appear like mats of wet wool. They are single cells that link to create long filaments or threads.

The algae occur in natural surface waters, where they appear as green or in some cases brownish, yellowish, or grayish.

In the first stages of growth, the filaments will attach to walls, plants, and other objects. They will eventually detach to form loose-floating mats. The algae do not have roots, and they rely on the water for nutrients. The causes of the algae include:

  • Nitrates – Aquatic plants need a steady supply of nitrogen, and they will stop growing once this demand is unmet. The algae will overrun your tank if the plants are stunted and not consuming any nutrients. A simple water test will reveal the nitrate content of your tank, and you can use fertilizers to boost the levels without adding any other nutrients.
  • Light – Excessive light when compared to the plant mass, may trigger the proliferation of the algae. Assess whether you have changed your aquarium’s lighting system to determine if the lighting is the problem. Increasing the light intensity means that you have to boost the supply of nutrients. Check if the position of the sun has changed because of a shift in seasons. Most aquariums need a maximum of 10 hours of light if they are under moderate to intense light. For tanks receiving low light, 12 hours of light will be sufficient.
  • CO2 – A CO2 deficit can also result in a filamentous algae outbreak. Using CO2 in your aquarium will greatly reduce the algae growth, however you need to dose fertilizers in order to maintain plant growth. In a pond or lake, using CO2 is not effective to eliminate algae.

How to Remove Filamentous Algae?

Before attempting to address the filamentous algae in your aquarium, it is recommended to identify its cause. In addition to light, CO2, and nitrates, fish waste can also promote algal growth.

Ensure that you are giving your fish just the food they need, and there is no additional food accumulating in your tank. Filamentous algae also love new aquariums, since there is little competition for resources.

Filamentous algae can be addressed in a number of ways, including:

Remove Filamentous Algae by Hand

Remove Filamentous Algae by Hand

Remove Filamentous Algae by Hand

This method involves the physical removal of floating clamps of the algae with a rake. If you are removing the algae from a pond, use a pond vacuum to trap the smaller algae strings.

Colorants and dyes are commonly used when you want to limit the amount of light getting to the algae, thereby slowing the plant’s rate of growth. These products include Crystal Blue and Aquashade. Keep in mind that dyes may affect the natural food chain of the system. The dyes are also not useful in water less than two ft in depth.

Other devices will work by chopping or cutting the algae. Remove the fragments to discourage the algae from re-growing. Raking and harvesting of filamentous algae will only yield short-term results.

Kill Off Filamentous Algae with Chemicals

Kill Off Filamentous Algae with Chemicals

Kill Off Filamentous Algae with Chemicals

Ingredients like copper-based compounds and sodium carbonate peroxyhydrate are used in the treatment of filamentous algae.

The chemicals should be dosed in the right amounts to deal with an outbreak of the algae successfully. The dosage will typically be given in gallons or pounds per acre-foot of water.

Copper compounds can be toxic to aquatic fish if dosed above the instructed amounts. Even if used in the recommended concentrations, the compounds can be dangerous to add to acidic or soft water.

It is, therefore, best to adjust your aquarium’s alkalinity before using copper-based solutions.

Copper Sulfate ranks as the most recommended chemical when dealing with filamentous algae. It is affordable and readily available in several forms.

The effectiveness will depend on how it is ground and the condition of the water. Smaller crystals are better than large ones, while it is not recommended to use the ingredient in very hard water.

There are also chelated copper products that are not considered very toxic to fish, although they should be handled carefully in aquariums with goldfish or trout. Some of the products will be sold with surfactants added to them to make them more efficient in controlling the algae.

Some will have gelatinous coatings, while others will be equipped with thick cell walls. You can also purchase and add a surfactant, but it must be rated safe for aquarium use.

Alkylamine salts of endothall are other ingredients used to address filamentous algae. The salts can be sourced in either granular or liquid formulations. They will rapidly kill any plant cells, but they can be toxic to fish.

There are several aquatic algaecides on the market, including Green Clean and Hydrothol 191. These ingredients are fast-acting, and they will eliminate all plant cells. Check if the algaecide is marked safe for the kind of fish and other live plants you have in your aquarium.

Diquat Dibromide is another contact herbicide, but it will only eliminate some species of the algae. You can either dilute it or use it as purchased, and it should be injected below the surface of the water.

You can use a hand-pump sprayer to apply the herbicide to your aquarium. The risk with chemical control is oxygen depletion because of the decomposition of the algae. Proper aeration should remedy this problem.

EasyLife AlgExit is another great product that can help eliminate filamentous and other types of algae in your aquarium. In combination with liquid carbon such as EasyLife Carbo or Flourish Excel, you can achieve really good results in fighting algae.

While most chemical are really effective in killing algae, I still believe that the best way to combat them is with natural remedies, which are harmless for the environment and fish.

Seek Help from Nature to Handle Filamentous Algae



A number of biological methods will also deal with the filamentous algae in your aquarium. Barley straws have been shown to reduce algal growth, where about three to five straws are used in a surface acre of a pond.

The bales are separated first before the straws are bundled in loose groups and submerged with a cloth or wire. The results on the use of barley straws are, however, not consistent.

Another kind of biological management is encouraging the growth of planktonic algae. These algae inhibit water clarity by allowing less light to reach the aquarium’s bottom.

A reduction in light intensity will greatly hamper the growth of filamentous algae. This solution is, however, very intensive to use, since fertilizer additions will need to be well-scheduled to maintain the blooms.

Tilapia has been shown to consume filamentous algae. This fish lives in warm waters, and it will not bear temperatures below 55 °F (12 °C). These fish might not be suitable for small aquariums, but in ponds will do a great job eating filamentous algae.

Grass carp is often quoted as a way to control filamentous algae. This fish does not prefer the algae, although it will be consumed in small amounts if there are no other plants. The algae will continue to thrive if they are other plants to be fed on.

Another biological process is reducing external sources of nutrients that can sustain the growth of filamentous algae. If you have a pond, ensure that it is far from sources like agricultural and lawn fertilizers or structures like septic tanks.

Is Filamentous Algae Harmful to Fish?

In natural ecosystems, filamentous algae oxygenate the water and is an important base of the food chain. An excess of the algae, however, is more harmful than it is useful. Pond owners commonly report extensive mats of these algae covering large portions of their ponds, especially over Spring.

If they cover the surface of your aquarium, you will have fish kills and low-light penetration. A bloom will also cause bad odors due to poor oxygenation. If the algae die, it will leave behind a decomposing mulch of organic matter which will contaminate your aquarium.

Should I Remove all Algae?

We have already established that filamentous algae are a natural part of any aquatic system, but how much is too much? Pond owners can leave some algae between June and September when the water is warm.

Filamentous algae should not cover more than 20% of any water surface since it will result in fish kills and stagnancy. The treatment of the algae in ponds during drought is not recommended. There will be an increased risk of worsening the water quality at this time, and there will also be higher chances for fish kills.

Why Does Filamentous Algae Produce Bubbles?

The algae release oxygen in the course of photosynthesis, which will be trapped underneath the mats of the plant, and cause them to rise. Bubbles are also formed which rise to the water’s surface. A pond’s surface may seem to bubble if there is an outbreak of the algae.

Is Filamentous Algae Harmful to Humans?

Most species of filamentous algae do not release toxins that are harmful to people. The presence of algae may, however, point to the presence of other pollutants in the water. If the pollutants are animal waste, pathogens and other bacteria can be trapped in the algae’s mats.

If you must handle the algae, it is best to use a sanitizer and wash your hands thoroughly. In the wild, the algae are designed to protect water quality by trapping contaminants. Even if the sight of the algae is unsightly, it poses little threat to your health.


Filamentous algae appear as mats floating on the water’s surface. Aquarists know it better as pond scum, as they can easily overrun a pond and aquarium. In aquatic ecosystems, the algae oxygenate the water and even provides food to aquatic inhabitants.

In ponds and aquariums, however, the algae can result in stagnancy and clots as well as fish kills. You can eliminate the algae with various chemical, mechanical, and biological controls. The most effective methods involve the use of chemicals, although it is vital to be keen on the dosage to avoid contaminating your aquarium.

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