Gill Maggots (Ergasilus) Fish Treatment Guide 2023 [Updated]

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How to Treat, Prevent & Identify Gill Maggots In Fish 2023 (Updated)

Ergasilus is a large genus of small crustaceans, well suited to both marine and freshwater habitats and found around the world. With most Ergasilus species, the adult females become ectoparasites of fish, attaching primarily to the gills, while the adult males are largely planktonic and harmless to fish.

Over time, gill maggots make fish significantly more vulnerable to secondary illnesses and diseases such as columnaris, koi fungus, and infected skin lesions. As such, it’s exceptionally important to regularly monitor your water quality, and identify and treat gill maggots as soon as possible.

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What Are Gill Maggots? Are They Dangerous?

Ergasilus gill maggot in fish pond
Female Ergasilus sp. X40. Source: Research Gate

Gill maggots are the adult females of all species within the Ergasilus genus. The males are planktonic, meaning that they float about harmlessly and feed on tiny bits of organic debris in the water. Though gill maggots more commonly plague aquaculture fish species like koi, they can also infect wild fish species such as pike, rainbow trout, and bream.

Females attach to the gill filaments (and occasionally fins) via two sharp, claw-like antennae structures with pincers that cause significant tissue damage. Serrated tooth-like structures surrounding their mouth enable them to feed on epithelial cells, blood and mucus continuously.

Each parasite is typically over one millimeter in size and easily visible to the naked eye. Their body appears like a tiny scorpion, but behind them trail two long sacs of white or greyish eggs, each of which look quite like a maggot, hence the name “gill maggot.”

Where Do Gill Maggots Come From?

Gill maggots find their way into ponds from newly acquired fish that haven’t been properly quarantined prior to introduction into the pond. If they were bred in an overstocked aquarium or fish farm, the likelihood of fish having parasites is fairly high due to poor water quality. Always quarantine new fish for several days prior to putting them in your pond, and observe them for any signs or symptoms of gill maggots or other parasites.

Additionally, larval gill maggots can be transported from live food. If you purchase live food, such as bloodworms, tiny shrimps, etcetera, for your fish, be sure to only purchase them from a reputable and trustworthy source, and check them very thoroughly for any parasites. Use a pond microscope if possible.

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What is the Life Cycle of Gill Maggots?

Gill maggots on fish gills
Female gill maggots infect the gills of fish, which eventually hatch into larvae, known as nauplii.

For a parasite, the life cycle of gill maggots is a relatively long one. After mating, female gill maggots immediately attach to the gills of a host fish. They will develop two long trailing “tails” of egg sacks that closely resemble maggots. Each clutch can contain over 200 eggs. These eggs typically hatch within a week.

These parasites prefer warmer waters, ideally those 24° C (75° F) and above, though this depends somewhat on the exact species. The majority of Ergasilus eggs hatch during the height of summer, when development is hastened by warm temperatures and eggs can hatch in as little as 3 days and develop into adults in as little as a few weeks. Once temperatures fall below 15° C (60° F), development into adulthood takes closer to 3 months.

After hatching, Ergasilus larvae, known as nauplii since they are copepods, float about freely, simply feeding on tiny bits of floating organic matter. This primarily consists of algae. They go through three to four developmental stages as nauplii, and then several stages as copepodites, before reaching reproductive maturity.

Reproduction typically occurs in the spring, with males dying after mating and females attaching to a fish host to feed and lay eggs. This process can occur as many as five times per year per female gill maggot, meaning that a single gill maggot can give rise to as many as 1,000 young per year. Females are also able to overwinter on fish, if needed, and typically live for a year or less.

What Are Symptoms of Gill Maggots in Fish?

Female gill maggots on fish gills carrying eggs sacks
Andreas R. Thomsen / CC BY-SA

1) Damaged Gills

The most obvious sign, next to seeing the gill maggots themselves, is gills that are damaged. They can become yellow or red, tattered, swollen, and coated in mucus. This is because gill maggots excrete digestive enzymes that digest tissues externally before being consumed by the gill maggot, resulting in great damage and pain to your fish.

2) Epithelial Hemorrhaging

Oftentimes, attachment sites will hemorrhage and may also develop painful, red blood blisters.

3) Hyperplasia

If gill maggots have been attached for a while, hyperplasia may occur at attachment sites. This means that gills and surrounding skin will become noticeably swollen. Pink/red discoloration is common as blood cells charge to the skin’s surface. In severe cases, gill filaments may even fuse together.

4) Difficulty Breathing

For obvious reasons, fish with significant gill maggot infestations may have a hard time breathing properly as the gill filaments will have a reduced ability to absorb oxygen as they normally would.

5) Flashing

In an effort to relieve their discomfort, fish with gill maggots may flash and flick about or rub themselves against rocks and the sides of the pond.

6) Secondary Infections

In addition to being able to see gill maggots, you may also notice that your fish have developed secondary ailments due to their weakened state. Bacterial and fungal infections, such as fin rot and koi fungus, aren’t uncommon.

Best Gill Maggots Treatment & Prevention Methods

If you suspect that a fish may be ill, or notice that it’s already infected with gill maggots, quarantine it from the rest of the pond immediately to hopefully prevent the spread. Treat as needed in a quarantine tank, and keep a close eye on the rest of the pond.

1) Salt Baths (ParaSalt)

Salt baths help rid koi of gill maggots

Gill maggots are incredibly hardy and difficult to treat. Your best bet is to quarantine all affected fish and completely drain and clean your pond, allowing it to dry for several days to ensure that all larval and adult gill maggots die.

Once quarantined, first try a 3% salt dip for no more than 10 minutes. Then try a .3-.5% bath for 3 days, increased up to 6 days if gill maggots are still alive.

2) Copper & Ferric Sulphate 

Copper and ferric sulfate to treat gill maggots in fish

If this doesn’t work or there are a lot of gill maggots, the Fisheries and Aquaculture Journal recommends treating your fish in a bath of a combination of .5 ppm copper sulphate and .2 ppm ferric sulphate for 6 to 9 days.

Keep in mind that both of these chemicals can be quite harmful, and so should be dosed very carefully and fish should be monitored closely for any signs of further distress. Ferric sulphate in particular is very toxic, and so we’re hesitant to fully recommend this as a treatment option.

3) Dipterex (Dylox)

Dipterex, also sold under the names Dylox, Neguvon, and a variety of others, has been found to be effective in treating gill maggots. Its active ingredient is trichlorfon, a white crystalline solid that is often sold in powder form. Treat in the form of a 6 hour bath at a concentration of .15 ppm.

4) Quarantine New Fish

New fish should be quarantined in a tank to prevent spread of gill maggot parasites

Prevention is the best medicine! First and foremost, quarantine any new fish before adding them to your pond. Closely monitor them for several days, checking for any visual signs of gill maggots. You should inspect for any other parasites as well, using a microscope if needed to look at water and skin samples.

5) Improve Water Quality

Aerators improve water quality and deter pond parasites

As always, check your pond water quality at the same time each day and adjust the temperature, pH, KH, and so on as needed. Perform daily 10% water changes or weekly 20-25% ones (daily water changes are better, as they place less stress on fish). Clean out any floating debris such as foliage on a daily basis. Make sure to have a quality filter, aeration system, and a variety of submerged and emergent plants to help naturally oxygenate, filter, and cool the water.

6) Provide Good Nutrition

Proper diet reduces the likelihood of health issues developing in koi

Feed your fish a well-balanced and highly nutritional diet to boost their immune systems and subsequently their ability to fend off and heal from any potential parasitic threats. You can purchase a high quality pellet feed, supplement with fruits and veggies as needed, or make your own pellets!

Be sure to research the fish species that you have and what their nutritional requirements are prior to purchasing or making any food.


1 thought on “Gill Maggots (Ergasilus) Fish Treatment Guide 2023 [Updated]”

  1. The maggots accommodate one gill only and they look like ‘real maggots’ that you see on rotting meat. The Koi is still eating well and isn’t rubbing itself against anything. It doesn’t appear to be in any distress, it isn’t presenting with any different behaviour, so is there any advice you can give, thanks.


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