aquarium pests

Aquarium pests / 12 species

It happens that aquariums are invaded and colonized by “pest” organisms that cause fears among aquarists, although they are harmless by nature. Sometimes these concerns are not unfounded since the presence of these organisms in large numbers (which is of the nature of an epidemic) is a sign of deterioration of the fish habitat. In addition, this indicates a lack of attention to detail on the part of the aquarist: he does not check live food in which they may be present and collects decorative material in natural reservoirs inhabited by fish, which should not be done. Such pests are unpleasant to look at and create certain inconveniences. Although they do not directly harm fish, their presence is a warning signal and should not be ignored.

Asellus water hoglouse

Asellus water hoglouse

Asellus aquaticus is an aquatic crustacean also called waterlouse. They bear some resemblance to the Woodlouse, of which they are related. They can be brought into the aquarium along with living food (or live food), and they populate inaccessible crevices in decorative objects, as well as the filter. They do not directly harm the fish, but they can act as intermediate hosts for Acanthocephala (thorn-headed worms). These worms rarely infect aquarium fish, so there is no need to worry too much about this. However, the large population of Asellus aquaticus indicates that there is a lot of organic pollution in the aquarium, to which these creatures contribute further.

Polychaete worms

Polychaete worms

These are annelids from the family Naididae, reaching a length of 2 cm. They are characterized by the presence of bristles (perhaps due to these bristles they are inedible for fish). Most often these are white or pink worms that reproduce by budding or laying eggs. They can be added to the aquarium with the plants, in the shells of aquatic snails, or in the water used to carry wild-caught live food. In an aquarium, they can live in the soil layer or on its surface, feeding on detritus. Therefore, their presence in significant quantities is an indicator of poor aquarium hygiene and the danger to fish due to pollution. By themselves, these worms are harmless. Improving the hygiene of the aquarium will keep the quantity under control and will benefit the fish by improving living conditions.


Algae are lower aquatic plants that, depending on the species, either attach to underwater surfaces or live freely in the water. They can be of different shapes and colors (green, brown, red, gray, yellowish). In appearance, they can resemble a slimy coating or fluffy tufts, form a carpet resembling moss, or long fibrous tufts (for example, the ooze that we see in ponds in summer). Real algae are different from so-called blue-green algae, which are scientifically classified as cyanobacteria.


The presence of algae is inevitable where water, nutrients, and light are simultaneously present. All three elements are found in every aquarium, so the aquarist must learn to accept that algae is a natural and inevitable part of the aquarium ecosystem. In an aquarium, as in nature, algae offer a variety of benefits. They are natural food for some herbivorous fish. Together with the microorganisms that live on them, they are an excellent source of first food for fry. They reduce the amount of nitrate in water, which is used as food. In addition, algae give completely naked decorative objects, such as stones, a more natural look, because, in natural bodies of water, stones are usually covered with algae. If the algae grows too vigorously, this is a warning of possible water quality problems.

Different aquariums are stocked with completely different types of algae. This depends in part on the lighting in the aquarium, with low lighting encouraging the development of brown slimy algae and bright lighting encouraging the growth of bright green algae.

Algae are believed to be particularly troublesome when they cover aquarium glass and plant leaves. However, it is not difficult to keep the front glass of your aquarium clean with a scraper or magnetic glass cleaner. Algae can be left on glass panes that are not used for viewing. By remaining there, the algae will help remove nitrates and provide food for the fish. If the aquarist has a liking for photography, the algae-covered back glass is an added advantage because it prevents the light from the flash from reflecting.

In nature, it is completely normal for algae to cover the old leaves of aquatic plants. Such leaves die off and are successively replaced by new leaves. If algae is causing problems, it is usually too fast algae growth or insufficient growth of higher plants. Feeshkeepers often make the mistake of reducing the intensity or duration of lighting in an aquarium in an attempt to contain algae growth. But instead, they inhibit the growth of higher plants and thus further exacerbate the problem! Healthy plants help inhibit algae growth by competing for available nutrients.

If it becomes necessary to remove algae from fish tank glasses every few days, it means that algae is growing too vigorously. This phenomenon occurs when there is an excess of nutrients. Algae is becoming a real problem – this is a sign of high levels of nitrogen or phosphorus. This, in turn, maybe due to too high a dose of aquarium fertilizer designed to accelerate plant growth but is more often an indicator of poor aquarium maintenance – overcrowding, overfeeding fish, insufficient water change, excessive waste, or a combination of these factors. The tap water used for partial water changes in the aquarium may contain too much nitrate and other substances that serve as food for algae. Thus, it is not the algae themselves, but the cause of their excessive growth, whatever it may be, that requires attention! Otherwise, the health of the fish will be adversely affected – due to contamination of the aquarium, and not due to the algae themselves!


Free-floating algae, which are invisible to the naked eye, sometimes multiply to such an extent that the water becomes cloudy and resembles pea soup. This is the so-called “water bloom”. This phenomenon can be observed in ponds in summer, but it can also be in an aquarium if it is illuminated for a long time by bright sunlight. And this problem is also an indicator of a high level of organic pollution.

Although there are special aquarium products on the market for killing algae, the solution to this problem, which is too easy to look at, is not really a solution. The death and decomposition of large numbers of algae can overload the filtration system and further exacerbate the pollution problem that has caused the algae to proliferate. Even if this does not happen, dead algae will further increase the organic matter in the aquarium. Therefore, when the algae repopulates the aquarium (and this will inevitably happen), then the problem will become even more acute than the first time. Repeated use of algae control chemicals and biological overloading will almost certainly have adverse effects on fish and higher vegetation. Therefore, it is better to establish and eliminate the cause of the excess amount of algae, and if their growth is normal, consider them more friends than enemies.

Some fish, such as the Gyrinocheilus aymonieri and some sucker-mouth catfish, are known to be “algae eaters” and can be used to inhibit algae growth. However, this does not obviate the need to keep organic waste in the aquarium low.

Copepod crustaceans

Copepods are small aquatic crustaceans. Most of them are harmless to fish. Some free-living species, such as Cyclops, are used as live food. At the same time, representatives of some species of copepods parasitize fish.

Free-living, harmless copepods are usually translucent and reach a length of 3 mm. They move in short leaps, but can also lie on underwater surfaces, including on the glass of the aquarium, where they are brought either intentionally (as live food) or accidentally (on plants). Few manage to survive long in an aquarium – for most fish, this is a real treat. True, large fish do not pay attention to them – after all, they are too small and should not be eaten. Thus, contamination of the aquarium with free-living copepods can only occur if the fish do not eat them – either because they are not the right food, or because the fish feel so bad that they have lost interest even in such a tempting food source. … This may be due to environmental pollution (heavy organic load). If copepods begin to breed in an aquarium, then organic pollution is taking place.

If the problem that caused this behavior of the fish is eliminated, then the fish will be happy to solve it themselves.


This is a group of microorganisms that cause the growth of a substance that resembles algae. Aquarists call it “blue-green algae“. The appearance of such “algae” is associated with high levels of nitrates and phosphates. True, not all aquariums with a lot of organic waste are filled with this “algae”. In one night they can cover all decorative objects in the aquarium, including the soil, with a slimy bluish-green bloom. There is no evidence that they directly harm adult fish (but they can be harmed by poor water quality, which has caused rapid reproduction of cyanobacteria). However, fry lying on the ground or decorative objects, these “algae” can very quickly cover and suffocate. In addition, they can completely cover the plants and destroy them.


It is very difficult to get rid of blue-green algae completely. Subsequently, at the slightest deterioration in water quality, they can begin to multiply rapidly again. The only way out is to reduce the amount of organic waste and filter out as much of this green mass as possible during the next partial water change. Unfortunately, blue-green algae seem completely tasteless to fish. They say that these algae feed on ground Red-rimmed melania. In addition, these snails create as much inconvenience as the cyanobacteria themselves.


These small coelenterates are freshwater relatives of sea anemones. In length, they can range from 2mm to 2cm (including tentacles). They are in the form of a stem, topped with tentacles at one end, while the other end attaches to a solid base. All these signs make it possible to unmistakably recognize them. True, sometimes they shrink into tiny jelly-like balls. Their color can vary from cream to gray or light brown. (There are hydras of a pleasant green color, which can easily be mistaken for algae.

Look at how hydra tried to catch shrimp!

Hydras (Hydra) sometimes enter the aquarium along with live food or decorative objects collected from nature. Subsequently, they settle on some objects or aquarium glasses and represent additional interesting objects, almost as enchanting as the main inhabitants of the aquarium.

For adult fish, hydras are safe, but they can catch fry and other small fish, as well as small particles of food intended for fish. Sometimes their numbers reach such a level that they become real pests. Like many other pests, they indicate problems associated with maintaining the aquarium.

To completely destroy the hydras, you have to completely empty the aquarium, scrape all its surfaces, rinse gravel, decorative objects, and underwater equipment in hot 2-5% saline solution at temperatures above 40 ° C. If the aquarium is planted with plants, then these plants are unlikely to take cleaning well in hot salt water! Therefore, it is better to use an alternative method, which consists in removing all fish (as well as snails, if they are the desired inhabitants of the aquarium) from the aquarium in some temporary room and raising the water temperature in the aquarium to 42 ° C for half an hour. During heating, the filler that serves as a substrate for bacteria should be removed from the internal filters, but the filters themselves are best kept there because the hydras attach to their surface. External filters should be turned off, but not for more than an hour, otherwise, the bacterial population may die due to lack of oxygen. Then the aquarium should be allowed to cool down to normal temperature, or it should be cooled by a partial water change by adding cold water. Then you put the fish (and snails) again and restore the filtration.

In an aquarium stocked with fish, the hydra population can be controlled by dissolving table salt in the water to produce a 0.5% saline solution. Such a solution should be maintained for about a week, and then gradually get rid of the salt through multiple partial water changes. This method should only be used if all fish tolerate this salinity well. Otherwise, you will have to regularly clean the glass of the aquarium, filter out the detached hydras, and remove stones and other hard decorative objects from the aquarium and subject to processing in hot salt water.

Some species of fish feed on hydras (especially gourami, as well as young cichlids “grazing” on rocks). Therefore, they can be used to control the hydra population, but only if these fish are suitable inhabitants for the aquarium in question.
Some species of fish feed on hydras (especially gourami, as well as young cichlids “grazing” on rocks). Therefore, they can be used to control the hydra population, but only if these fish are suitable inhabitants for the aquarium in question.



Leeches are accidentally brought into the aquarium and can be seen on decorative objects or floating freely in the water. Representatives of some species of leeches parasitize fish.

Roundworms (nematodes)

nematodes in the aquarium
They are a large group of filamentous worms, also called roundworms. Among them, there are both free-living and parasitic species. Nonparasitic nematodes are red-brown worms with an unsegmented body 1-3 cm long. Sometimes they colonize the substrate and biological filter. They can be added to the aquarium along with live food and are completely harmless. If they become too numerous, this indicates that adjustments to the hygiene of the aquarium and the feeding regime of the fish are needed. These improvements are all that are needed to reduce the nematode population.

Ostracod crustaceans

ostracod cleaning crew!
The shell crustacean Ostracoda is a bean-shaped crustacean that reaches a length of 4 mm. Sometimes you can see them scurrying around on the substrate, like tiny moving specks. These creatures are yellowish or black-brown in color. They attach their eggs to plants, so they can be accidentally brought into the aquarium along with the plants as well as live food. They are found in small numbers in aquariums, but if the hygiene of the aquarium leaves much to be desired, they can begin to proliferate and become a real disaster. Thus, although barnacles are harmless, their presence indicates a problem with the environment or with the fish’s diet. These problems can have adverse effects on fish. Improving aquarium care is a solution to both problems at the same time. It allows you to control the number of these animals and eliminate the causes of their rapid reproduction.


Planarian are harmless, non-parasitic flatworms that often strike fear into the hearts of hobbyists by mistaking them for parasites like leeches. They usually reach 2-10 mm in length and are creamy white, gray, or brown in color. Light planarians appear translucent on the glass of the aquarium, while dark planarians look like tiny slugs. Characteristic features are a V-shaped head and slow gliding motion over the surfaces of the aquarium.

Planarian can be inadvertently introduced into the aquarium along with aquatic plants or live food. In an aquarium, there may be a small and completely invisible population of planaria living in the ground or on its surface. Sometimes their numbers become too large, and then you can see how they crawl on the front glass of the aquarium or swim freely in the water. Such rapid reproduction is an indicator of the overfeeding of fish. The uneaten food is consumed by planaria, whose population is growing at an explosive rate. It is necessary to review the feeding regime of fish and improve the quality of water if it has suffered from overfeeding.


The planarian problem often occurs where large fish feeds on food. After that, a whole rain of food particles flies out of the gills and falls to the bottom. These particles are too small to be of interest to large fish. In such cases, it may not be the quantity, but the type of feed, which is the real problem. A possible solution is to change your diet or keep fish in the aquarium, which has a habit of digging in the ground and cleaning up anything that falls from above.

Some fish species, such as gourami, eat planaria and thereby control their numbers. However, this control method does not avoid other hazards such as contamination from uneaten food. Therefore, it should not be seen as an optimal solution to the problem.


Some aquarists deliberately introduce aquatic snails into the aquarium to act as “cleaners” and to clean up food debris. Sometimes snails enter the aquarium by accident – usually on plants. But no matter how the snails get into the aquarium, if later they turn out to be undesirable, it is not at all easy to remove them from there. This is especially true of the viviparous snail called Red-rimmed melania (Melania tuberculata). These snails live in the ground, where they can reproduce very intensively, and in such a way that the aquarist has no idea that they are there.

The presence of a large number of snails is an indicator of an unhealthy aquarium. For snails to live, they must not only find enough organic matter to feed but also consume oxygen and produce organic waste. Some of them devour fish eggs. Snails caught in the wild or caught in ornamental ponds can introduce a variety of parasites into the aquarium, for which they are intermediate hosts.

The snail population can be kept within reasonable limits by regularly removing any snails that can be seen. For example, you can collect them with a net or filter them. Large individuals can be caught individually by hand. It is best to remove snails after turning off the lights in the aquarium, as most snails are active at night. Ground snails leave it at night to look for food on decorative objects and glass of the aquarium.

There are special products on the market for killing snails, but their use in aquariums inhabited by fish is undesirable. Most of these shellfish products contain copper, which is toxic to fish, so overdosing can be fatal. Snail carcasses can heavily pollute the aquarium, especially where Red-rimmed melania is the main problem. Even if this chemical is applied at night, countless dead snails are likely to remain in the substrate. If you are nevertheless convinced that such a remedy must be used to kill sand snails, the fish should be transferred to another room. After that, you need to process the aquarium, then thoroughly clean it and reinstall it, filling in a new substrate.

To avoid the accidental introduction of snails, it is perfectly acceptable to treat them with a shellfish killer before planting them in the aquarium. Remember, tiny snails may be overlooked when visually inspecting plants. Do not buy fish from aquariums with Red-rimmed melania. If they get into any of your aquariums, keep it in quarantine until you achieve their complete destruction, because tiny newborn snails can very easily end up as “stowaways” in nets, fish bags, siphon tubes and others items of equipment.

Tubifex Sludge worm

Tubifex Worms In Wastewater
The tubifex is commonly used as live food for fish, although they can introduce disease into the aquarium. If it is thrown into an aquarium that contains soil, some of them may burrow into it and avoid being eaten. This can also happen if the fish are fed from a special feeder and too many tubulers are fed at one time. Those sludge worms that remain uneaten will eventually crawl out of the trough and fall to the bottom. As a result, a whole colony of tubules is formed in the soil – small red-brown worms, partially protruding from the substrate. The best solution is to stop feeding the fish. When the fish get hungry due to the lack of easier prey, they will take the trouble to catch these worms and will soon solve the problem themselves.