Parasites can be a real problem for pond fish. Here's what you need to know about Pond Parasite.
Prevention
Parasites are introduced into ponds from other fish, so with careful precautionary measures you can avoid parasitic infestations. Before you introduce a new fish to the pond, it should be quarantined for three weeks during which time you should treat it for parasites. Sometimes symptoms don't appear right away so if you don't treat the fish during quarantine there is no guarantee that the fish will not bring parasites into the pond. Also be careful where you purchase your fish. Only purchase them from a trusted, reputable dealer.
Symptoms
If a fish is isolating itself, has stopped eating, seems to be gasping for air, is sitting at the bottom of the pond with its fins pulled in tightly or has sores on its body or fish are dying, its very likely parasites are the culprit.
Diagnosis
The most common pond fish parasites are flukes, costia, chilodinella, trichodina and ich. All these parasites cause the common symptoms listed above. In addition, costia and cholodinella are characterized by excess slime production, trichodina often causes a gray film to develop on the fish and ich produces white salt-like granules on the fish's scales. Flukes can cause fins to fray and the skin to redden.
Treatment
Treatment will depend on the type of parasite your fish has. If you're unsure, take a scraping to your vet and have her look at it under a microscope to determine the problem. Commercially prepared medicinal treatments will have your fish back on their fins in no time.