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Ich: An Old Cure for an Old Disease

By Terry Ranson
From Vol. 2, No. 2, The Newsletter of The Tri-State Aquarium Society, January 2000

Probably the most common disease among fish is ich. But, what do you really know about this organism?

Ich is short for the name of a ciliated protozoan of the genus Ichthyophthirius. Ich is usually present all the time in aquaria in small numbers, just like germs are in the air we breathe. However, when a fish suffers from extreme stress, such as a sudden drop in temperature, its resistance is lowered and it becomes vulnerable to diseases. Ich outbreaks also occur after the introduction of new fish to an established aquarium.

Ich is free-swimming until it attaches itself to the skin of a fish. Under a microscope, the organism is easily seen and identified, even under low magnification. It looks like a round, rolling mass. According to John Gratsbek, et al, in the book Aquariology, The Science of Fish Health Management (Tetra Press), ich is one of the few fish parasites completely surrounded by cilia. The organism's U-shaped nucleus is often visible under a microscope.

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Once the free-swimming ich reaches a fish, it attaches to the outer layer of the skin of the host fish. The ich organism then forms a tough outer shell, or cyst, while it feeds on the fish's bodily fluids. This encysted stage, called a theront, grows large enough to be seen with the naked eye. Each theront appears as a tiny white spot on the fish. Severe ich infestations make fish appear as if they are covered with salt. After theronts grow to a certain size, they break through the skin and drop off the fish. As they fall, they attach to the bottom or sides of the aquarium, or to plants, gravel, decorations, tubing or any other stationary object. Theronts then begin their reproductive stage, and are then called a trophozoite, also known as a trophont. The attached trophozoites then begin producing the infective, free-swimming stage. Hundreds more free-swimming ich organisms, called tomites, can arise in less than a day and a half, and they in turn re-infect the fish in your aquarium.

In nature, ich is not much of a problem. There are large numbers of fish to which tomites can attach. And with the greater amount of water volume, it's likely that many ich organisms do not even find a host. However, in a closed system like an aquarium, ich re-infects the same fish over and over, resulting in severe infestations. That's why it can be such a problem.

While ich is encysted on the fish, no medicine can affect it. But once it's free-swimming, it can be killed. Since the life cycle of ich takes at least three days at 80 degrees to complete, ich must be treated for at least four days. I prefer to treat for a week.

Although many aquarists use rather harsh chemicals to kill off Ich, I prefer more natural methods:

If your fish recover from ich, they may not get it again. There is evidence that fish become resistant to ich after they survive the initial infection, so fish which recover from an ich infestation should be less likely to contract the disease at a later time. However, I would still recommend a three-week quarantine period for all newly purchased fish.

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