Garden pond water parasites can quickly kill any koi fish or gold fish in your pond. Fortunately, most garden pond parasites can be destroyed by adding salt to the water.
Costia is a common parasite that attaches itself to the gills and skin of garden pond fish. It affects not only koi and gold fish but other species of fish, both saltwater and freshwater. This parasite can't survive for more than a few hours in a garden pond that doesn't contain fish because it can't survive without a host for very long. This type of parasite has the appearance of a comma, and can also infect the fish internally where you can't see them.
Costia kills fish quickly, so if the number of fish in your garden pond is decreasing, you may have a costia problem. The fins of your koi or gold fish may look reddish and they may seem like they're having trouble breathing and almost gasping as they do so. The fish may also have extra mucus on their skin, or markings that look like a spider web.
Chilodinella attack freshwater fish and can kill gold fish quickly. These are heart-shaped and will attach themselves to the gills of gold fish. Fish will roll on their sides and start to float as they struggle to breath. Adding a .03 percent mixture of salt to the water will kill both costia and chilodinella.
Ich, also known as White Spot, is a common parasitic infection that fish can get in garden pond water. The first sign is usually small white spots on the fish that look like grains of sand. Fish may swim close to rocks, plants and gravel in what appears to be them scratching against them. As it worsens, fish seem lethargic and swim very little, and may even start to look red or bloody. The white spots may spread making it obvious that Ich is to blame. This parasite burrows into the skin rather than attach to the gills. Special medication is required for the water to kill Ich.
Argulus, also known as fish lice, typically attacks fish in dirty ponds or mud ponds rather than in garden pond water, but it can still happen. These are visible on the fish, because they're the biggest parasite found on fish. They usually appear as greenish or brownish spots. They usually attach themselves to the gills, but can be found on the skin as well and kill fish by devouring too much of their blood or by causing an infection. Dimilin or brand name treatments kill the parasite.