[untamed]

This is the old argument AGAINST quarantining. Unless your QT is a healthy environment, you are correct in that your QT may be worse for the fish than adding it directly to your healthy tank.

The key is that by QT'ing you are protecting your existing fish from anything that the new fish might introduce. That is mainly what it is about.

It doesn't sound good for your fish. Maybe someone can recommend a medication that could be used since he is already isolated. (I've never successfully medicated a fish) Meanwhile, you need to do a lot of water changes on that QT to keep that ammonia level low.

I don't think that they symptoms you describe would be dangerous to other fish if you chose to place him into the display tank. Unfortunately, even healthy fish have a tough time being the new guy. An unhealthy fish rarely survives being the new fish in the tank.