Sorry, that is very frustrating. I went through five or six fish like that. I finally gave up, drained the QT
, and haven't tried since. I am however planning on giving it a new try very soon.
If it is a penguin it has a biowheel. They are very good when established but intermittent use can contribute to a high ammonia spike due to bacteria being established then dried up or not used.
Most times it is nothing that you are doing wrong.
On the positive side, if it had a parasite, or fungus, at least it is not in the main tank!