Koi ponds sometimes present different challenges from aquariums. For example, sometimes koi ponds face large external parasites that rarely hit aquarium fish. These include gill maggots, which can harm or kill koi. However, you do have several different options for the prevention, treatment and eradication of these parasites. As with many problems, prevention can often require fewer resources and less time than cure.
Gill Maggots
Gill maggots (Ergasilus spp.) are relatively common in pond fish, but rare in aquariums. They have no relation to real maggots. Instead, these parasites belong to the crustacean clade. However, they do superficially resemble maggots, having large, soft egg structures. These parasites attach themselves to koi's gills, and are visible when the fish flaps its gill covers during respiration.
Effect of Gill Maggots
Gill maggots have several effects on koi. First, their feeding weakens the fish. Koi may get so weak that they have trouble feeding, and become emaciated due to starvation. Additionally, the parasites can interfere with the koi's ability to breathe. Lastly, gill maggots leave koi vulnerable to secondary infections. In secondary infections, normally harmless bacteria and fungi take advantage of a fish's weakened state. This allows them to gain a foothold and cause infections, further harming the fish.
Prevention
It takes less effort to prevent gill maggots than it does to treat them. First, you can inspect the koi for gill maggots before purchasing them. Simply look at the gills as the koi respires. Unlike many fish diseases, gill maggot infestations are somewhat obvious. You should also quarantine your koi for at least two weeks in an aquarium before adding them to a pond. If you purchase your koi from a pond, you may not notice parasites on the gill and body, since these may not be as obvious from above. You can also take the step of a saltwater dip. Just get a bucket and add 2.5 to 4 ounces of aquarium salt per gallon of bucket water. Add the koi for 15 to 30 minutes, then place them the pond. Koi can tolerate salt better than parasites can.
Treatment
If you already have gill maggots, you need to take several steps to treat them. First, you will need to treat the infected fish. Catch any fish with gill maggots and treat them with a saltwater bath; then remove the parasites directly. Hold the fish immobilized in a wet towel and seize the parasite with a pair of tweezers. It should come right off, but if it doesn't, give the fish a few more minutes in the saltwater bath and try again. You will also need to treat the pond itself to catch any free-living parasites. Use an organophosphate-based pesticide. Pet shops that sell pond supplies will sell appropriate pesticides.
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