Gill flukes in fish: signs, praziquantel treatment and why you treat twice

Gill flukes are tiny parasitic worms that hook onto a fish's gills, causing fast breathing, flashing and gill covers held shut. The most important thing to do is rule out bad water first, then treat with praziquantel at least twice, because the first dose doesn't kill the eggs.

Flukes are flatworms in a group called monogeneans. The best-known gill fluke family is Dactylogyrus and its relatives; a close cousin, Gyrodactylus, lives mostly on the skin and fins. Most are smaller than 1 mm, too small to see without a microscope. They pass directly from fish to fish with no other animal involved (UF/IFAS), so they build up fast in crowded tanks. Gill flukes lay sticky eggs that land in the gravel and filter, resist medicine, and hatch days later. Skin flukes give birth to live young that already carry the next generation, so numbers can explode. At 22–25°C (72–77°F) the whole cycle can take only a few days (UF/IFAS).

How to identify it

Breathing faster than normal, while tankmates breathe calmly.
Flashing: rubbing the head or gill covers on rocks or gravel.
One gill cover held closed, or gills flaring oddly.
Clamped fins and reduced appetite.

Easily confused with:

Ammonia or nitrite poisoning and low oxygen. Test the water first.
Velvet: look for gold dust with a flashlight.
Ich on the gills.
Overheating.

A vet or fish health lab can confirm flukes with a gill or skin snip under a microscope.

When it's a problem

Early: one or a few fish breathing fast, flashing, or holding a gill cover shut. Treat today.
Serious: swollen, pale or mucus-coated gills; puffy gill covers; weight loss, pale colour, skin sores, and secondary bacterial or fungal infections.
Emergency: "piping", which is gulping at the surface, especially when oxygen is low (Merck Veterinary Manual), and slow, steady losses over days to weeks.
Normal, don't worry: many fish carry a few flukes without harm. Numbers rise and gills get damaged when fish are crowded or stressed.

What to do right now

Test ammonia, nitrite, pH and temperature. Rule out water problems before you blame parasites. Gasping in poor water is the more common emergency.
Add aeration. Flukes damage gills, so give the fish as much oxygen as possible. Add an air stone; aim the filter outlet at the surface.
Do a 25–30% water change and vacuum the gravel. This removes some eggs.
Remove activated carbon from the filter before medicating.
Reduce crowding if you can, by moving some fish to another established tank.
Get a praziquantel-based product from a fish store, or ask a vet.

What to do next

Praziquantel (first choice)

The Merck manual calls praziquantel, as a prolonged bath, the treatment of choice for monogeneans in ornamental fish. It kills adult and juvenile flukes but not the eggs. That's the key to getting it right:

Treat at least twice. The second dose catches flukes that hatched from eggs after the first. UF/IFAS describes repeated or prolonged treatment for egg-laying gill flukes, with re-examination 1–2 weeks after treatment to decide whether to repeat.
The exact dose and timing differ by product. Follow the label. Many products dose once, then again about a week later.
Praziquantel dissolves poorly. Mix it well, as the label says. UF/IFAS warns that badly prepared powder can harm gills.
It can break down fast in established tanks, and activated carbon removes it.
In some countries (including the US), praziquantel for fish isn't formally approved and may need a vet; elsewhere it's sold for aquarium use.

Praziquantel is generally considered gentle on fish, plants and filter bacteria. It is reported to be safe for most shrimp and snails, but it kills planaria and other flatworms, and product formulations vary. Read the label.

Other options

Formalin is used in fish farming and in some aquarium products. It's harsh on sick fish and needs strong aeration. The Merck manual notes eggs resist it, so it's repeated weekly.
Salt dips (short, strong baths) remove some skin flukes, but do not remove eggs and aren't safe for every species. Low-level salt in the tank is not a reliable fluke cure, and UF/IFAS cites research where slightly salty water actually increased one type of skin fluke.
Organophosphate pesticides are old fluke treatments. Avoid them in home tanks: they're dangerous to people and to many fish.

Cautions

Plecos and other catfish, loaches: generally tolerate praziquantel, but watch closely after dosing. The Merck manual notes that, unlike most skin flukes, those found on plecos (the family Loricariidae) can lay eggs, so give plecos the same repeat-dose approach.
Shrimp and snails: check the label. Formalin and many "general cure" mixes harm them.
Sick fish tolerate formalin poorly. If fish react badly after any dose, do a large water change.

How long it takes

Breathing should improve within days of the first dose. Complete the second dose even if the fish look better: the eggs in the gravel don't care how the fish look. Damaged gills take a few weeks to heal, so keep water clean and stocking light.

Products that help

Medication containing praziquantel: the first-choice fluke treatment. Plan for at least two doses, as the label directs.
Liquid test kit for ammonia, nitrite and pH: rules out bad water, the more common cause of gasping, before you treat for parasites.
Air pump and air stone: extra oxygen for damaged gills.
Gravel vacuum: removes some fluke eggs from the bottom during water changes.
Thermometer: to rule out overheating as the cause of fast breathing.

Keep on hand before trouble starts: a praziquantel product, a water test kit and an air stone. Many keepers also use praziquantel routinely in quarantine.

How to prevent it

Quarantine new fish for at least 4 weeks. Many keepers give a praziquantel course in quarantine as routine, especially for wild-caught fish, plecos and goldfish.
Don't overstock.
Keep water clean and stable: weekly tests, regular changes.
Use separate nets per tank.

Why it happened

A new fish brought flukes in. UF/IFAS notes some fish carry them without showing disease.
Crowding let numbers build up. Flukes spread best through packed tanks.
Stress (poor water, aggression, temperature swings). Stress hormones have been shown to boost fluke reproduction (UF/IFAS).

When to get help

Get help if fish keep gasping after you've confirmed the water is fine, if two courses of praziquantel didn't help, or if you're losing valuable fish. A vet who sees fish can do a gill snip on a live or freshly dead fish (flukes decompose fast after death, so bring the fish quickly, chilled not frozen). Honest limits: a fish with badly damaged gills may not recover even after the flukes are gone.

In the game

In Multiple Tank Syndrome, the Gill flukes card says "tiny worms on the gills spread fast through crowds": every group of 3 or more fish in the hit tank dies. The Praziquantel card or the Medicine basket saves the tank. Real flukes do the same thing: crowded schools are where they thrive.