Filamentous algae are green algae in long filaments that invade ponds and lakes as soon as the water is too rich in nitrates or phosphates. They mainly appear in spring and summer, favored by excess sunlight and insufficient filtration. To act quickly: remove them manually, add Aquipond Greenstab, and test your water before any chemical treatment.
At a glance, it's hard to miss. Filamentous algae form long green strands, sometimes sticky, that look like hair or green cotton floating in the water. They cover the walls, cling to plants, and eventually form real mats on the surface.
Typical visual appearance:
Bright green filaments, sometimes brownish-yellow at the end of the cycle
Cottony or slimy texture depending on the species
Floating clumps or attached to walls and stones
The two most common species in our ponds:
Spirogyra: fine, unbranched filaments, slimy and viscous to the touch. This is the most common species in calm freshwater. Its massive presence often indicates organic or mineral pollution.
Cladophora: branched filaments, rougher to the touch. It attaches to stones and submerged substrates using rhizoids and tolerates harder water (pH 7 to 10).
Filamentous algae vs green water: what's the difference?
Green water is caused by microscopic suspended algae (planktonic algae). It makes the water cloudy and greenish. Filamentous algae, on the other hand, are visible to the naked eye as strands or clumps. Both can coexist, but the solutions differ: a UV lamp eliminates green water but remains ineffective against filamentous algae.
Filamentous algae thrive when three conditions are met: nutrients, light, calm water. Here are the most common causes.
Excess nitrates and phosphates
This is the number one cause. Fish waste, uneaten food, dead leaves decomposing at the bottom: all of these release nitrates and phosphates into the water. These nutrients feed algae. A nitrate level above 30 mg/L or phosphate level above 0.03 mg/L promotes their growth.
Insufficient or poorly maintained filtration
A clogged filter no longer properly treats the water. Beneficial bacteria that break down organic waste disappear, and nutrients accumulate. Result: algae take over.
Lack of aquatic plants
Oxygenating plants and marginal plants consume the same nutrients as algae. Without them, filamentous algae take over first. The recommended biological balance: plants should cover about 1/3 of the pond surface.
Excessive sunlight
A pond exposed to full southern sun all day heats up quickly. Direct light stimulates algae photosynthesis and accelerates their growth, especially between May and August.
Analyze your water first
Before any treatment, do a water test. Measure at least: pH, nitrates, phosphates, and KH (carbonate hardness). A test strip kit is enough for an initial diagnosis. If nitrate or phosphate levels are high, no algae treatment will be sustainable without correcting these parameters at the source.
This is the approach we favor at foudebassin.com: treat the causes, not just the symptoms. These methods are safe for fish and pond wildlife.
The [barley straw for ponds][LIEN INTERNE paille d'orge] is one of the best-known and most used natural solutions. As it decomposes in the water, it releases small amounts of hydrogen peroxide that slow the growth of filamentous algae.
How to use it:
Introduce from the end of winter or early spring, when the water reaches about 10°C
Indicative dose: 10 to 15 g per m² of water surface (or 200 to 300 g per m³ for concentrated granules)
Place in a net or bag, near a waterfall or pump to diffuse the active compounds
Replace every 4 to 6 months
Important point: barley straw works better as a preventive measure than as a curative treatment. If the pond is already overrun, combine it with manual algae removal.
Beneficial bacteria break down organic matter (fish waste, dead leaves) and thus reduce the amount of nutrients available for algae. They are added directly to the water, as a spring treatment or after a chemical treatment to reseed the biological filter.
To prioritize:
At the beginning of the season (March-April), when the filter bacteria are still not very active
After each partial water change or chemical treatment
Oxygenating plants like elodea (Elodea canadensis), hornwort (Ceratophyllum demersum), or water crowfoot (Ranunculus aquatilis) consume nitrates and phosphates directly from the water. They deprive algae of their food.
Water lilies and floating plants play a complementary role: by covering the surface, they limit light penetration and slow down algae photosynthesis.
Some fish graze on filamentous algae directly on walls and stones. Herbivorous carp (Ctenopharyngodon idella) are the most effective over large areas. Be careful not to introduce too many: in overpopulation, they unbalance the pond in turn.
There are several solutions to limit or even eliminate the presence of algae in your pond. It is important to act as quickly as possible, using the right method. If you delay taking action, it will be more difficult to get rid of them.
Algae tend to develop especially at the beginning of the season, when aquatic plants are in vegetative rest. During the season, aquatic plants compete with algae and reduce the amount of nutrients available to algae. The first thing to do in spring to limit algae is to restart your filtration system.
Next, the second thing to do is to carry out a water analysis with the appropriate equipment. You will need to check several parameters:
If necessary, also check the phosphate and nitrate levels. You can purchase a test kit (see photo below), which will allow you to analyze these different parameters in a few minutes.
It is important to check GH+ & KH+ to balance the pH. Good water hardness (GH & KH) also acts as a protective barrier for the fish in your pond. You need to "mineralize" your pond after winter because rainwater significantly lowers water hardness. A natural pond is generally constantly fed by springs, which helps maintain high water hardness.
⭐ Important: Before carrying out an algae treatment (which you can find here), it is essential to check the hardness (GH and KH) of the water of your pond. This treatment should not be done if hardness levels are not corrected.
⭐ You might also be interested in: our guide for analyze the pond water
What are the causes of filamentous algae development in a pond? Algae can appear for several reasons in a water body. In most situations we encounter when visiting our clients' ponds:
We will need to distinguish several things: internal and external factors
First of all, know that filamentous algae ensure the balance of your pond or lake. But their presence has significant consequences, both negative and positive. If these algae are present, it is often for several reasons:
When water temperatures are too cold, it is sometimes unavoidable to treat the algae. If the water is below 12°C, you should avoid anti-algae like Algisin.
The development of filamentous algae occurs naturally, even in a difficult environment. Their appearance is explained by the presence of three factors:
Even if you have poor quality water, the development of filamentous algae is always possible. This is sometimes also the case in good quality waters.
To fight against the proliferation of filamentous algae, you can choose different types of solutions (available right here) :
Eliminating filamentous algae is good. Preventing their return is better. Here are the actions that really make a difference.
Balance the pond long-term:
Cover 1/3 to 2/3 of the surface with aquatic plants (water lilies, floating plants, oxygenators)
Adjust fish population to water volume (minimum 50 L per goldfish, more for koi)
Feed fish the right amount: food should be consumed in less than 5 minutes
Seasonal maintenance:
Spring: add beneficial bacteria, introduce barley straw, clean the filter
Summer: monitor water parameters weekly, manually remove the first clumps of algae as soon as they appear
Autumn: place a net to catch fallen leaves before they decompose in the water
Winter: clean the bottom of the pond, reduce fish feeding
Other good practices:
Place the pond in partial shade if possible (avoid full sun all day)
Check and clean the filter regularly, without sterilizing it completely (to preserve beneficial bacteria)
Test the water at least once a month during the season
____________________________________
If you do not want to use chemical products, it is perfectly possible to choose natural solutions to eliminate filamentous algae.
First, pond dredging is necessary, involving removing plants using an excavator. A particularly effective solution, because not only will you get rid of filamentous algae, but also all the nutrients that feed them.
Be cautious during this process by avoiding leaving algae near the pond. If heavy rains occur, the algae could end up back in your pond.
Beyond using the excavator, it is recommended to use beneficial bacteria like Aquipond bacteria. Use living organisms aimed at eliminating filamentous algae from the pond. Indeed, it should be known that beneficial bacteria can feed on excess nutrients present in the water.
Thanks to their action, the algae will gradually die. Beneficial bacteria do not aim to kill these algae but rather to regulate them. For effectiveness, it is necessary to follow the instructions. For example, avoid combining them with herbicides, otherwise the beneficial bacteria will die very quickly.
⭐ You might also be interested in: the list of oxygenating plants for a balanced basin
Another technique is to use ultrasound. By using the resonance of ultrasonic waves, you can effectively eliminate harmful algae. This is a proven process based on very serious scientific research.
Specifically, you need to get a transducer, fully submerged in your basin. This device will be able to generate sound waves that penetrate the gas vacuole. Consequently, filamentous algae will not survive.
This solution, very effective, remains costly and its effectiveness is limited on ponds. The action of ultrasonic waves must always be adapted according to water quality to ensure optimal action against plants.
Some fish like carp will help regulate algae in your basin. However, carp will be of no use if you specifically want to combat filamentous algae.
If you have unwanted plants, you can also use barley strawIt is a particularly interesting alternative to herbicides. Indeed, thanks to its presence, the pH level will then drop as will the carbonate hardness of the basin.
Under these conditions, you have water with better balance and you control the growth and proliferation of filamentous algae much better. As soon as barley straw comes into contact with water, it produces peroxide. A substance known for eliminating unwanted plants.
Below are some related articles that might interest you:
In small amounts, no. They are part of the aquatic ecosystem and even serve as shelter for some microorganisms. However, massive proliferation can be problematic: algae consume oxygen at night (during decomposition), which can cause O₂ deficiencies and stress fish. They can also block pump strainers and disrupt filtration.
The most effective method combines several actions: manual removal of visible clumps, adding barley straw from spring, planting oxygenating species (elodea, hornwort), and introducing beneficial bacteria. These solutions act more slowly than chemical treatments (expect 2 to 4 weeks), but their effects are long-lasting.
Because the root causes have not been addressed. If the water remains too rich in nitrates or phosphates, if filtration is insufficient, or if the pond lacks plants, algae will systematically return after each treatment. The key: analyze the water, identify the imbalance, and correct it at the source.
The herbivorous carp (Ctenopharyngodon idella) is the most effective. It feeds directly on aquatic plants, including algae. Other species like certain catfish or tench can help limit algae on the walls. Be careful to balance the population: too many fish produce too much waste, which worsens the problem.
Eaufrance - Waterbody quality - official water information portal in France
AquaForte - Optimal water values for a pond - guide to parameters to monitor
DORIS / FFESSM - Spirogyra sp. - scientific description sheet
Artois-Picardie Water Agency - Filamentous Algae - official datasheet on proliferating species
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