Camallanus worms are treated most effectively with levamisole hydrochloride added directly to the aquarium water at a dose of 2 to 13 ppm, repeated at least once to kill larvae that hatch after the first treatment. The full life cycle of the parasite takes about 21 days, so a second dose roughly three weeks after the first is essential to break the reproductive cycle. Without that repeat treatment, surviving larvae will reinfect your fish and you’ll be back where you started.
Recognizing the Infection
The signature sign is red, thread-like worms protruding from a fish’s vent. These are female worms releasing larvae into the water. If you can see them, the infection is already well established. Light infections are much harder to spot. You might notice a fish that eats well but looks bloated, or one that thrashes along the water’s surface as if trying to dislodge something. Chronic infections can cause a hollow belly appearance because the worms damage the intestinal lining and the fish can no longer absorb nutrients properly.
Camallanus is especially common in livebearers and cichlids. In cichlids, it often shows up as bloat, white stringy feces, and loss of appetite. If you’re seeing these symptoms in multiple fish, treat the entire tank rather than just the visibly infected individuals. The larvae are free-swimming, so any fish sharing the water has likely been exposed.
Levamisole: The Preferred Treatment
Levamisole hydrochloride is the most widely used and reliable treatment. It paralyzes the worms, causing them to release their grip on the intestinal wall so the fish can expel them. Recommended aquarium doses range from 2 to 13 ppm (milligrams per liter). Fish tolerate it well even at higher doses, with the lethal concentration for fish sitting around 250 ppm, far above any therapeutic dose.
To calculate your dose: for a conservative 2 ppm treatment in a 50-gallon (roughly 190 liter) tank, you’d dissolve about 380 mg of levamisole hydrochloride into the water. Dissolve it in a cup of tank water first, then pour it in. Leave it in the water for 24 hours, then perform a large water change (at least 50%) and vacuum the substrate thoroughly. Dead and paralyzed worms, along with larvae, will be sitting on the bottom.
Three weeks later, repeat the entire process. The 21-day life cycle means larvae released before or during your first treatment will have matured into new adults by then. Some fishkeepers do a third treatment three weeks after that for extra insurance, particularly in heavily infected tanks.
Invertebrate Safety During Treatment
Levamisole is often described as invertebrate-safe, but that’s an oversimplification. Shrimp and nerite snails generally tolerate it at the lower end of the dosing range (around 2 ppm). However, mystery snails and other members of the apple snail family are at serious risk. Many fishkeepers report losing all their mystery snails after a single levamisole treatment. If you keep apple snails, move them to a separate container before dosing. For tanks with sensitive shrimp species, stick to 2 ppm and don’t exceed it.
Fenbendazole as an Alternative
If you can’t find levamisole (it’s not always easy to source depending on your country), fenbendazole is the main alternative. It works differently, disrupting the worms’ ability to absorb energy, which kills them over several days rather than simply paralyzing them. Fenbendazole is most effective when mixed into food so the fish ingest it directly. You can blend the powder into a gel food or soak pellets in a fenbendazole solution.
The challenge with fenbendazole is that fish who are already losing their appetite from a heavy infection may not eat enough medicated food to get a therapeutic dose. For this reason, levamisole (which goes directly into the water and is absorbed through the gills) tends to be more reliable for advanced infections. If your fish are still eating normally, fenbendazole mixed into food works well and results are typically visible within a few days. As with levamisole, you’ll need to repeat the treatment to catch the next generation of worms.
Cleaning the Tank Between Treatments
Camallanus can complete its life cycle without an intermediate host in aquarium conditions, meaning larvae released into your tank water can directly infect other fish. This is why substrate cleaning matters so much during treatment. After each dose, vacuum the gravel or substrate thoroughly to physically remove paralyzed worms and larvae. Do large water changes (50% or more) to dilute any remaining free-swimming larvae. If you use sponge filters, give them a squeeze in old tank water to flush out anything trapped in the foam.
In the wild, the parasite normally passes through tiny crustaceans called copepods before reaching a fish. But in a closed aquarium system, that intermediate step is skipped entirely. The worms can reproduce and reinfect generation after generation without any other organism involved. This is why camallanus can spread so aggressively in a fish room, and why every tank that has housed an infected fish needs to be treated.
Supporting Recovery After Treatment
Killing the worms is only half the job. Camallanus worms physically attach to the intestinal lining, and when they’re expelled, they leave behind damaged tissue. That damage creates openings for bacterial infections to take hold. If your fish still look lethargic, stop eating, or develop new symptoms like fin rot or redness after the worms are gone, a secondary bacterial infection is likely.
During recovery, focus on three things: high water quality, low stress, and good nutrition. Frequent small water changes keep ammonia and nitrite from spiking, which is especially important if sick fish are producing less waste than usual. Feed high-quality, easily digestible foods to help fish regain weight. Frozen or live foods like brine shrimp and bloodworms are good choices because they’re protein-rich and most fish will eat them eagerly. Keep lighting on a normal schedule, avoid rearranging the tank, and minimize netting or handling fish.
If secondary infections are severe, a broad-spectrum antibiotic or antifungal may be necessary. Treat in a hospital tank if possible, since antibiotics can disrupt the beneficial bacteria in your main tank’s filter. Fish that were heavily infected and have developed a hollow belly or arched back may take weeks to recover full body condition, and some with extensive intestinal damage may not recover fully.