Parasites in Fish

There are many factors affecting fish health and survival rates. In this article, we will focus on some common parasites in the aquarium hobby and the treatment methods.

These parasites are most visible or have visible symptoms and make the fish suffer from pain and any possible induced infections.

Argulus(Fish Lice/ Fish Louse)


Argulus species are from the family Argulidae. They are commonly called fish lice which cause fish diseases and infections. They belong to a big group of branchiuran parasites but argulids are linked to lobsters, crabs, and shrimps because they are a type of crustaceans. More than 100 species of Argulus are found all over the world. Some species are more famous because they are mostly found on freshwater fish. The most common types are Argulus coregoni, Argulus foliaceus, and Argulus japonicas. Usually, carps, minnows, goldfish, salmonids, centrarchidaes, and koi fishes mostlythe victim of this infection. Argulus foliaceus attacks toads and frogs, too.

Argulus has flat and oval or some species have a round. They have two compound eyes and a sharp needle-like probe at the mouth. This is used for piercing the fish body and sucking its body fluids. Until the lice grow to the adult phase. They grow two suckers on both sides of their mouth apart from the probe. The hook-like suckers are used to attach to the host’s body. They also have spines at their rear end which together with suckers help them stay attached to the fish.

Adult Argulus are almost 3 to 7 mm long and 2 to 4 mm wide. Some species can be larger. Females are bigger. Each female has a seminal receptacle on its back which receives sperm from the male.


The infestation of Argulus reaches its peak in fall and summer. You can see the lice with naked eyes attached to the skin of the fish. The gills chamber and mouth also get infested. The areas where the lice attached are inflamed and irritated because of the digestive juices of the parasite. In case of heavy infestation, the entire body of the fish including fins can be covered with lice.

Prominent symptoms in a mild infestation are not very specific. You may find fin and scale loss, pinpoint hemorrhages, anemia, spots lethargy, increased mucus creation, erratic swimming, hanging at the surface and reduced appetite. You can see the overall body condition of the fish poor and weak. In more serious infestation, the fish get irritated and may rub or flash against the surface to feel relieved or remove the lice. The overall look of the fish indicates sickness.

A secondary infection might also erupt or localized inflammation causes damage to the area.  The parasite reproduces fast and the damage on fish skin disables the fish to balance internal mineral salt level and body fluids. Bacteria and water molds like Saprolegnia and Aeromonas often attack the fish that is infested with Argulus by taking advantage of the weak immune system. In this case, the fish may die of excessive bacterial growth in its body.

Argulus Life Cycle

The life cycle of Argulus is simple and direct. It needs one host to develop from an egg to a fully grown adult. But their growing process is complex. The outer surface called the exoskeleton is made of a material called chitin. They shed this shell in their growing process and grow a new one. This is called the molting process. Argulus larvae go through 11 to 12 stages of molting while growing.

Adult Argulus can also survive away from the host for several days. After mating, the female leaves the body of the fish and searches for hard surfaces or vegetation’s to lay the eggs. After laying the eggs, she finds another host fish and gets attached to it. The young Argulus take 30 to 40 days to reach a mature adult age being hooked with the body of its host.

Argulus also carry several diseases from one fish to another fish. They transmit a viral disease called: spring viremia of carp. This disease mostly spreads between koi, goldfish and common carp. Argulus can also work as the intermediate host for numerous species of roundworms (nematodes).


When you find Argulus on the fish in an aquarium (visible with the naked eye), choose the treatment of the fish according to the life span of Argulus which spreads over 30 to 60 days. Treatment should be good for eliminating all age stages from egg to adult Argulus. Move the fish into a clean tank and treat them in the new clean environment. Disinfect the old tank properly.

For bath treatment of the fish in a treatment tank, chose the suitable medicine from the many available. When you buy the medication for Argulus, try to identify first and mention the species with the level of infestation. Using organophosphate pesticide for prolonged treatment usually in a heavy infestation is effective. The other option is Diflubenzuron for the treatment. This pesticide disturbs the molting system of the parasite which results in its elimination. It is also effective for killing all the eggs, young and adult parasites. Check the label for instructions and method of use carefully.

Do not use formalin or salt for eliminating Argulus as it is not effective. And do not mix infested fish with other healthy species without completing the treatment.

Gill Maggots


Ergasilus is a crustacean type parasite. The females of the species are called gill maggots. The colors of these are white and grayish-black with a few millimeters long body. They are found both in seawater and freshwater. They usually enter from the mouth of the fish and infest the gills. Some of them get stuck to the inner cavity of the mouth as well. Gill maggots are commonly found in pond fish and rare in aquariums but sometimes available. They resemble maggots in their appearance. Gill Maggots become visible while the fish flaps its gills for respiration.

Gill maggots are adaptive to the migratory lifecycle of salmon fish. Atlantic salmon is the most infested fish by this parasite. They are not found in sea trout, brown, parr or smolts. Gill maggots carry two egg sacs each containing around 900 eggs attached to the behind of the females. These look like long tails.

Life Cycle

Ergasilus has a lifecycle of three stages. In the Nauplius stage, it has six moltings. In the Copepodid stage, it has an additional five molts. Then it reaches the adult stage. The dorsal organ existing in the naupliar stage gets lost in the later molts of the copepodid stage. In the late development of adults, the male maxilliped appears and becomes fully functional in the final molt of the adult stage. Females have no maxilliped.

The female carries the egg sacs behind her like her tail. Once the eggs are ready, she drops them to the water bed. They hatch at the bank and swim freely in the water. This period is spread over a few days and at this time, they have to find a host and attach to it to survive. Most often they use salmon fish as a host. They are swallowed by the host fish and then eventually, they get stuck with the gills with the help of a filament. After attachment, the maggots start growing into an adult. The females stay there attached to the edge of the gills forever. They feed on the gills which are richly provided with blood.

After attaching to the gills, the gill maggots take several months to reproduce. They mature often in a year and start breeding.


A low level of the infestation does not cause much harm to the host fish. If the infestation level is high, the damage can be extensive. The gill structure starts losing its tips and scar tissue develops which later may cause the death of the host.

But koi fish has more severe effects of gill maggots’ infestation. The fish becomes weak because of the maggots constantly feeding on their blood. The weakness may cause koi fish to stop feeding and starve to death. The infestation can make the respiration for the fish more difficult. The outcome of a heavy infestation in koi can result in secondary infections. Secondary infections include fungi and bacterial attacks that leave the fish weaker and more prone to weakness and illness.


Prevention of gill maggots is easier than treatment. That is why take some steps to prevent your koi fish from suffering from gill maggots. You must quarantine your newly bought fish for two weeks before adding them to the aquarium. If you buy your koi from a pond, the effects of gill maggots on its gill won’t be obvious. Just place your koi in saltwater for half an hour. Add 2.5 to 4 ounces of salt to each gallon of water (~4 liters). The parasite dies from salt but fish doesn’t get affected.

If your fish develops gill maggots, you need to treat the infected fish accurately. Give the infested fish saltwater bath and then hold them with a towel and remove the maggots with tweezers. The paralyzed maggots come off immediately but if not, give your fish another saltwater bath and then remove the maggots. The pond can also be treated for gill maggots with organophosphate based pesticide.



Septicemia is a serious blood infection. It’s also known as bacteremia or blood poisoning caused by gram-negative bacteria. These bacteria release toxins that activate the immune response and blood coagulation inside the blood vessels. This eventually reduces blood supply to the organs and tissues. But this process does not happen unless there is any wound on the body and the wound gets infected. Following the infection in any part of the body, in case the infection also enters in the bloodline it appears.

Usually, fish becomes infected with Septicemia when it gets wounded as a result of fights or hits against the hard/sharp structures in the aquarium. The high nitrate and nitrite levels in the water provide a suitable environment for bacteria growth which increase also the possibility of Septicemia to spread in fish.

Another trigger for Septicemia is feeding the fish with bacteria-infected food. Foods that borne pathogens are dangerous for the fish. Live foods like tubifex worms and blackworms should be clean of any bacteria. Wash the live food daily or grow your own culture for safe feeding.

Frozen food is also a carrier of these bacteria when you leave the frozen food thawed for a day and then feed it to the fish. A day-old thawed frozen food develops bacteria in it. Do not feed it to the fish to prevent any kind of possible infection.


Examine the body of the fish for Septicemia. You can notice redness under the scales at different places on the fish’s body. It may be scattered all over the body or can be concentrated at a specific part. Often it appears around the head. The redness is difficult to locate if the fish is of dark color or in red color. In such cases, examine the wounds on the fish’s body. If it is pinkish and clean it will cure soon but if it is black or gray or fuzzy or the surroundings are discolored, it means that the fish is suffering from Septicemia. It needs instant treatment with antibiotics.

Another symptom is the appearance of a secondary infection in the fish. It can lead to Popeye when one or both eyes bulge out. It may cause dropsy, too. Other symptoms are loss of appetite, sluggish behavior, color loss or clamped fins. If you observe your fish closely every day, you can easily find out the Septicemia in its early stages which will also increase the treatment success.


Viral hemorrhagic septicemia (VHS) in fish is a deadly infectious disease which the viral hemorrhagic septicemia virus (VHSV) is the cause. It affects freshwater fish of more than 50 species and saltwater fish in several parts of the northern hemisphere. VHS is instigated by viral hemorrhagic septicemia virus (VHSV) and it has many different strains of which are present in different regions and afflicts different species.

Viral Hemorrhagic Septicemia virus stays active in cool water – temperature ranging from 5°C to 20°C. The ideal temperature is 10°C and the virus can survive outside the host for several weeks. The virus transmits from fish to fish directly when they come into close contact or from contaminated water. Reproduction of fish also transmits the infection to the newborn fish. It stays on the surface of the egg and then gets to the progeny.


Septicemia is an infection that is fully treated with antibiotics only because this infection is in the blood of the fish and can overcome the fish very fast. Antibiotics are the fastest and most effective way of treating Septicemia. The usual way of treating a sick fish is to dissolve the medicine in the aquarium water and expect the fish to absorb it. But Septicemia is an internal infection and this type of treatment doesn’t cure the fish quickly.

The best effective method is to use medicated fish foods if the fish were not lost their appetite. Soaking fish food in a mixture of antibiotic solution and feeding them to the infected fish is another option. But, this method is also not fully effective since you cannot know how much of it the fish consumed.

If the loss of appetite is high in the fish, it will not consume the medicated food and medicated foods in this case will not be effective. So, adding the antibiotics in the tank water will be the last choice for personal treatments. Injections can be also done by a Vet if the disease level is high and treatment is urgent.

It is difficult for a hobbyist to determine the kind of bacteria and level of infection in the fish. So, the best thing to do is to combine wide spectrum antibiotics while treating the infected fish. This way treatment will be more sufficient for both gram-negative and gram-positive bacteria.

Here are some effective medicines for treatments:

Aquarium Pharmaceuticals’ Triple Sulfa
Mardel’s Maracyn and Maracyn-Two in combination
If your fish has stopped eating from the loss of appetite, give it Kanamycin Sulfate. It works effectively.


Fish infections can develop a strong resistance to antibiotics and become strong enough to defeat antibiotic effects. Flavobacterium columnare is such a bacterium which is a transitioning bacterium. It has many strains and some of them are mild while others are severe. The newly known bacterium of this type in the aquariums is virulent and it kills the fish in one day. It has no treatment due to immunity to the antibiotics.

While using antibiotics you should avoid creating severe strains of bacteria mostly caused by the wrong use of medications. Use the antibiotics carefully and strictly as per the directions. Continue the same antibiotic for the treatment till the end of the prescribed time unless the fish shows an adverse reaction to the medicine. Always treat the sick fish in a hospital tank separate from the healthy fish. Never apply medication to healthy fish.

Blood Parasites (Trypanoplasma, Trypanosoma, Sanguinicola)


Trypanoplasma, Trypanosoma, Sanguinicola are single-celled microscopic animals. They are from a group of phyla of the kingdom Protista, like an amoeba, ciliate, flagellate, or sporozoan.

Blood parasites known in fish are Trypanoplasma, Trypanosoma, Sanguinicola. These internal parasites have a common name endo-parasites and they live in the tissues, organs like intestines, and blood. But some blood parasites are harmful while some are not for the fish. The most known blood parasite is Trypanoplasma salmositica (commonly referred to as Cryptobia samositica) introducing cryptobiasis of salmonids.

The type Trypanosoma includes various species of both freshwater and marine fish. Some freshwater species are pathogenic for cyprinids. Most cases of blood parasites exist in fish that are living in rivers and sea. Aquarium fish rarely gets infected with blood parasites.


The symptoms and signs of a blood parasite infection can be unclear in fish. An infected fish may not be socially active. It swims alone and mostly shows lethargic behavior or swims on one side. The movement of gills increases. The fins drop, clamp or lose shine. Fish also start losing weight because of not eating well and losing appetite. Tropical fish may sometimes rub their body to decorations like gravel, small rocks, or plants in case of infection. Sometimes fish also get bloat. In severe conditions, gills damage, exophthalmia, anemia, hepatomegaly, splenomegaly, abdominal swelling with ascites or kidney damage can occur. A surgical examination is the best method to determine the infection carried out by a specialized Vet or laboratory.

Life Cycle

The life cycle of blood parasite is complex and depends on the type of parasite and environmental conditions. Trypanoplasma and Trypanosoma have an indirect life cycle. If there are leeches in water, the life cycle is active because leeches for Trypanoplasma and Trypanosoma are the main vectors.


The treatment includes medicine that you can buy from any fish store. To make the treatment plan more effective, remove the leeches and snails from the aquarium. These blood parasites need a host and their best host is leeches and snails. If you leave the snails and leeches in the water with the fish, the treatment will not be effective. But do not remove the snails and leeches by killing them with medication because this can harm the fish and won’t eliminate the snails. Remove them manually for assured cleaning of the tank. You can pick them one by one with a tweezer or fishnet.

Anchor Worms (Lernaea)


Lernaea species are commonly known as Anchor Worms. They are copepod parasites, crustaceans that can infect, cause sickness, and kill several types of freshwater fish especially those found in ponds and rivers. The infestation with Anchor Worms is most prevalent in the summer months. And it occurs when the water is stagnant (it happens when the tank water becomes bad) or slow-moving natural water bodies. They are visible by the naked eye. The grown-up females look like thin ‘hair’ or ‘thread’ approximately 25 mm long. Under a microscope, the body looks tubular with an anchor at the anterior end and a pair of egg sacs on the rear end.

Up till now, 110 species of Anchor Worm or similar species are found. Lernaea cyprinacea is the most common species within all. It is commonly in goldfish, common carp, cyprinids, Koi, and many other freshwater fish like catfish and big-head carp. It can infect amphibians, too.


Lernaea infections show signs on the oral cavity, gills, fins, and skin. A severe infestation of lernaeids in their copepodid stages by badly damaging fish’s gills which makes respiration difficult to impossible.  The female Lernaea attaches to the fish by burrowing deep into the tissues and inserting an anchor like probe into the fish’s body. Intense hemorrhage and inflammation can happen at that point and the area looks ulcerated and red.

A mild infestation is not life-threatening but it can irritate the fish severely. Infection is inevitable and leads to secondary infections. And these secondary infections can kill the fish. Lernaea infections leave the health of fish extremely bad and its growth also gets hampered.

Life Cycle of Lernaea

Lernaea is from the family of crustacean copepods like lobsters, shrimp and crabs and they all spend a multi-stage life. It is a bit different among these because all copepods live freely and do not cause disease to the other aquatic animals. Lernaea has a complex life cycle. They do not need an intermediate host but they can directly spread from one fish to another. The direct life cycle takes 18-25 days until fully complete. One host, either a fish or amphibian is enough for the organism to reach from an egg to the adult stage.

After the parasite mate and the male dies. The female uses its large anchor at its head to get attached to the tissues of her host.  The female keeps growing to a more mature stage and lays the eggs within 24 to 36 hours. The eggs are released in a sac from her posterior and hatch into a nauplii stage within 24 to 36 hours. Females remain active and can produce 250 new juveniles every 2 weeks for up to 16 weeks especially if the temperature is warmer than 25°C.

Following the hatching, nauplii live freely and do not act as parasites. In four days they go through 3 stages of life. At this point, they start their first copepodid stage. They become a parasite and find a host and attach to its gills. Following 7 days it has 5 more different copepodid phases. After that stage the male mate and dies within 24 hours while the female keeps attached or changes the host. This entire cycle takes 18 to 25 days from egg to mature size.

The ideal temperature for Lernaea to live and reproduce is 26–28°C. Below this temperature, the nauplii of the species can not complete the larval cycles. At 14°C the female won’t reproduce and spend the winter attached to the host and produce eggs in spring.


The first and most important step is prevention to save the fish before a wide-spread and damage. It is best to quarantine the new fish properly and check for potential parasite threats in it before adding to the aquatic community.

Carps, koi, and goldfish are at high risk of carrying this kind of parasite. Observe accurately for parasites and continue monitoring them during the quarantine period. If any parasite is detected, treat them properly before moving to the main tank.

If your fish is infested with Anchor Worms, you can remove them manually with the help of forceps. But, this method is not hundred a percent fool-proof because the anchors remain embedded in the tissues of the fish in most of the cases. Removing a large level of infestation with forceps alone is not possible.  The best treatment is to use medicines that can target the entire life stages of Anchor worms and can eliminate the parasite.

The female Anchor Worms can survive 30 days on a fish and are hard to kill as compared to young ones. That is why it is strongly recommended to continue treatment of the fish for several weeks.

The medication differs from the fish species. You need to ask aquarium fish experts or a Vet to make sure the drug is effective on the parasite and safe for the fish.

Treatment with salt has shown the best success results in eliminating the Anchor Worms. But this salt treatment is not proper for some fish species and could be carefully searched and consulted with a specialist before application.

For the fish in an aquarium, additional drugs are necessary apart from the salt treatment. You need to leave the fish in water with an organophosphate such as trichlorfon. This is an effective treatment. To kill the larvae, bathe the fish for 30 minutes with 25 mg/L potassium permanganate.

Diflubenzuron is effective to kill both the molting adults and the larvae. Use a dose of 0.066 mg diflubenzuron/liter. If these drugs do not work, use 4.8 grams salt per liter for 30 days in case the fish can tolerate this level of salt. Remove all the fish from the tank for at least 7 days and run the tank without fish. This will break the life cycle of the lernaeid in the tank because they need a host to complete their life cycle.

Protect the Anchor Worm infested fish from developing secondary infections. Keep them in highly clean water during the period of treatment and observe their wounds closely to save them from any secondary fungal and/or bacterial infections.

Some Treatments and Dosages:
  • Potassium permanganate dip at 100mg/10 liters of water,
  • Saltwater dip,
  • Formalin dip at 2 to 4 ml Formalin/10 liters of water for 30 minutes. In case the fish lost equilibrium, immediately transfer the fish to clean fresh water.
  • Modern antiparasitics (such as Disco-worm, Fluke tabs, and Clout)
  • Salt in the aquarium at 1 to 2 tablespoons may help prevent secondary infections.

Image Source :
Crispin Rodwel
Dr. Mohammad Faisal

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