Quarantining new fish is a very good practice, but there is absolutely no guarantee that you can introduce them into your aquarium completely pest free. A lot of times, fish can show no signs of disease, parasite infestation, or other ailments. And in a QT tank, you don’t want to just medicate for everything on a fish that is apparently in good health, as medications can be quite harmful on an already stressed out fish. Because of this, you might have ich pop up on your new fish after introducing it to the display, or your existing fish may show signs of it. Unfortunately, it’s just one of those things that happens. Thankfully, a well QT-ed fish will likely take to prepared foods once in the display tank, even if it does show signs of parasite or disease, and it will be far better prepared to fight off whatever the ailment. Still, the hobbyist should keep a watchful eye on the new fish and all of its tankmates to ensure that the issue doesn’t get worse.