Have you noticed small, slippery white or gray worms on the glass of your aquarium or among the plants? Most likely, these are planarians. For most aquarists, their appearance is not a very pleasant surprise.
These flat worms are not just unsightly. They are predators and scavengers that can attack snails, their eggs, and even harm shrimp and their offspring. They are especially dangerous in spawning and shrimp aquariums.
The good news is that it is quite possible to get rid of planarians. There are several ways to do this: from folk remedies to special preparations. The main thing is to act wisely so as not to harm other inhabitants of your underwater world.
How to recognize a planarian
Planarians belong to flatworms, which means they are the most primitive representatives of their type. This makes them extremely adaptable and resilient. And while zoologists are fascinated by the characteristics of these organisms, for aquarists they are a real headache, because sometimes the only way to get rid of these pests is to completely restart the aquarium, disinfecting all components and quarantining the creatures.
Planarians range in size from 2 mm to 3 cm. Their bodies are elongated and flattened in the dorsal-ventral direction. The head has a clearly defined rhombic or triangular shape, and on closer inspection, two large black eyes can be distinguished, which the worms use to distinguish light from darkness.
They settle in secluded places – under stones, in grottoes, and in plant root systems. Their peak activity occurs at night. However, during the day, planarians often crawl onto stones, glass, plant leaves, or even rise to the surface of the water. Sometimes they swim with the help of small cilia covering the entire surface of their body.
Under normal conditions, planarians are whitish and translucent, but their bodies change color depending on what they eat.
What is the danger of planarians?
In terms of their feeding habits, these flatworms are predators. They do not penetrate fish and crustaceans like parasitic worms; their danger lies elsewhere. Their good appetite and high activity cause them to eat fish eggs. 
– especially snails and shrimp. However, the fish eggs of small species also suffer from planarians.
Adult animals are also not immune to the harm that these pests can cause. By clogging the gills of fish, planarians cause discomfort to the latter, and if there are too many of them, they can lead to suffocation.
The appearance of these flatworms is especially undesirable in shrimp tanks and other aquariums where crustaceans are present. The fact is that during molting, planarians penetrate the space between the old shell and the new, still delicate surface. Sometimes this can lead to the death of shrimp, especially small ones.
Not all aquarists agree on the danger of planarians to shrimp farmers. Planarians not only hunt, but also eat carrion. In a poorly maintained “bank” where shrimp often die and dead individuals are not removed in a timely manner, planarians breed intensively due to the large amount of food. The owner thinks that the presence of worms is the reason for the “death” of shrimp, that is, he confuses cause and effect. One way or another, planarians pose almost no danger to large adult specimens.
The situation is much worse with roe. Worms that have grown to 2 centimeters greedily eat it and also attack newly hatched shrimp. They can also attack adult snails.
How to remove unwanted inhabitants from an aquarium
A small number of planarians in a fish aquarium may be tolerable. In a shrimp tank or aquarium with mollusks, even a small number is undesirable. If the worms reproduce rapidly, they can spoil the appearance of the aquarium even in daylight.
In modern aquarium keeping, there are several ways to combat this pest.
Biological
In order to start the destruction process, it is necessary to introduce inhabitants into the aquarium that eat planarians. Unfortunately, not all fish will do this: many inhabitants will not touch these worms, as they secrete a special acrid mucus that is extremely unpleasant to the taste.
However, some organisms willingly eat pests, ignoring this feature:
Gouramis
Preference should be given to fry and newly grown fish, as they are particularly fond of eating planarians. A great advantage of this genus of fish is that they eat not only adult worms, but also eggs, which contributes to the complete eradication of pests when their initial numbers are moderate.
Melanotenia (rainbowfish)
This is a beautiful Australian fish that can be kept in both fresh and salt water.
1-Wrasse; 2-Cichlids; 3-Macropods
In large marine aquariums, wrasse will be useful. In freshwater aquariums, some cichlids and macropods will be useful.
1-Macrobrachium peguense; 2-Inlesee
Among crustaceans, Macrobrachium peguense and Inlesee shrimp can be considered enemies of planarians.
This method has its strengths and weaknesses.
Pros:
- there is no need to disrupt the water balance or introduce additional chemicals.
- no effort required on the part of the aquarist—the fish will do everything themselves.
Cons:
- it is not always possible to introduce a specific species of fish.
- the method is suitable for small numbers of worms.
Physical
Bait traps for planarians
The physical method has one goal—to remove all planarians from the aquarium, as destroying them one by one is an impossible task.
Traps are used for this purpose:
- Homemade traps are made from readily available materials, cost almost nothing, and can be made in any quantity.
- Factory-made traps can be found in specialty stores or ordered online. They can be quite expensive, but they last a long time and look neater.
The simplest trap is a piece of gauze in which you wrap a piece of bread or meat (e.g., chicken). The resulting lump is placed at the bottom of the tank overnight. In the morning, it is removed from the aquarium along with the worms inside and placed in boiling water, where the worms die within a couple of minutes. After that, the trap is ready for use again. This can be done two to four times.
Unfortunately, water from such a “tool” can become cloudy.
More complex traps are made so that it is easy to get in, but much more difficult to get out. These are often made by cutting a plastic bottle in half and inserting one part into the other, with the neck facing inwards.
Factory-made glass traps are even more sophisticated—they have several narrowing entrance tubes that are not so easy for dumb worms to climb out of.
Pros:
- minimal effort on the part of the aquarium owner;
- in the case of a homemade trap – low cost;
- reusable;
- does not affect plants and beneficial inhabitants.
Cons:
- ineffective against eggs – only works on adult specimens;
- the trap must be constantly removed and cleaned.
Chemical
This involves adding various substances or preparations to the water that have a destructive effect on flatworms and/or their eggs.
Life hack
If you want to use “chemicals” to combat planarians but don't trust special compounds, you can use what you have in any kitchen: table salt and table vinegar.
- Salt is added at a rate of 1 gram per liter—this concentration will kill most planarians.
- Vinegar is nothing more than a way to quickly and radically increase the acidity of the water. A weak 0.25% solution is added to the aquarium.
During such procedures, it is best to remove animals and sensitive plants and then quarantine them.
Experienced aquarists prefer ready-made preparations. Many of them have been tested by dozens of owners, but using strong chemicals is at your own risk.
Preparations and active substances that work against planarians:
These products are not specifically designed for aquariums—they are broad-spectrum anthelmintic drugs intended for warm-blooded animals, primarily cats and dogs. However, in the absence of specialized products, aquarists use them by dissolving the tablets in water.
Fenbendazole is not particularly dangerous for snails and aquarium shrimp. Before treatment, they must be removed. There are no official dosage instructions, so it is best to start with low concentrations. Experienced aquarists recommend a ratio of 0.2–0.4 g per 100 liters of water.
However, you can find stories online from individuals who have successfully used fenbendazole in shrimp tanks without harming the crustaceans.
User Zercon
Moscow
I used 2 tablets of “Kanikvantel Plus” for 500 liters of water. I dissolved them and poured them into the water. On the fourth day, all the planarians had disappeared. I don't have any shrimp, but I didn't remove any fish or plants. The biological balance is normal.
You can also find gentle products on the market. They are mainly designed for shrimp tanks, as these crustaceans are very sensitive to all kinds of “chemicals.”
No Planaria
Liquid solution of Katechu palm extract (correctly called Katechu acacia).
Benibachi Planaria Zero
Contains crushed betel palm nuts.
These preparations are of natural origin and do not pose a threat to the animal life in the tank.
Complete restart
Sometimes there is simply no other option—when there are too many planarians or you need to destroy the eggs of these pests. 
During the restart process, all non-living contents should be thoroughly washed and disinfected. Keep plants and fish in quarantine.
To prevent planarians from multiplying again, you should follow these simple requirements:
- do not overfeed the inhabitants;
- filter the water thoroughly.
- each newly introduced item (stone, grotto, decoration), as well as plants and animals, should be treated, kept, and quarantined before being placed in the aquarium.
Planarians are small white or transparent worms that appear in aquariums due to excess food, poor cleaning, or contaminated soil. They are not directly harmful to fish, but a large number of them is a sign of imbalance in the aquarium. You can get rid of them by improving care: feed the fish less often, clean up food debris more thoroughly, and change the water regularly. Sometimes natural methods help, such as introducing ramshorn snails to the aquarium, which compete with planarians, or using special preparations if the situation is serious. The main thing is not to panic and, first of all, to put your aquarium care in order.
Question and answer
What is planaria most often confused with?
Inexperienced aquarists may confuse small planarians with nematodes (and vice versa). The key feature is the flattened body shape and triangular head on one side of the planarian's body.
How do planarians appear in an aquarium?
Most often, they enter the aquarium in eggs, which are inconspicuous and resistant to many factors. However, adult planarians can also enter with aquarium plants.
Can planaria attack fish?
Even large planarians are too small for adult fish. However, larvae that have just hatched from eggs are at risk.
Can a small number of planarians be safe?
If you do not see these flatworms and cannot determine their presence by the behavior of fish or shrimp, then there is no cause for concern. However, if there is no roe or fry, even though there used to be, this is a reason to think that the planarians are eating the roe.
Completely removing planaria from an aquarium is not an easy task, so it is much easier to prevent the worms or their eggs from getting there in the first place by carefully monitoring everything new that is placed in the aquarium.
As you can see, planarians in an aquarium are an unpleasant but solvable problem. The main thing is not to panic when you find them. These worms signal that something has gone wrong in the established life of your underwater world.
The best strategy is always prevention. Regular care, feeding control, and quarantine of new plants and decorations will minimize the risk of these uninvited guests appearing. A clean aquarium is an uncomfortable environment for planarians.
If they have already settled in your aquarium, the choice of control method depends on your specific situation. For an aquarium with only plants or shrimp, traps and less aggressive drugs are suitable. In a general aquarium with fish, the range of remedies is wider, but caution is always required.
Remember that any treatment is stressful for the entire ecosystem. Carefully monitor the behavior of the inhabitants after using any of these methods. Patience and consistency are your main allies in this matter.
Ultimately, planarians are a temporary problem. Once you deal with them, you will not only clean the aquarium, but also establish a more stable and healthy environment for its inhabitants, which will benefit everyone.
