04-06-2012, 12:21 PM
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I agree with previous members. There is some debate among aquarists on ich, but there is sufficient evidence now to show that it can be "present" for months if not years without any visible signs. As soon as the fish are stressed by something, it will rapidly break out. This can happen in tanks with nothing new added for years, so clearly the ich has to be there. It is also generally present in all freshwater in nature, yet if you catch fish you won't see it.
An article in the current (April) issue of TFH by microbiologist Eric Hanneman describes this from his own experiences. He cured the tank of ich, and for several months the fish were fine. Then, suddenly in the winter the temperature in the tank dropped into the mid-60's F, and the fish were within a couple of days absolutely covered in ich.
I have had similar experiences with ich breaking out in tanks that have had nothing added for months. This is just one reason why preventing stress is so important. And stress can occur from many sources--inappropriate water parameters, sudden changes in parameters, ammonia or nitrite or high nitrate, too much light, inadequate environment, moving fish, using salt and chemicals, etc.