A lot of people learn this one the hard way

I think the advice you got in your other threads about this is good advice, and the links posted there will help you understand what happened. The idea is that you can't be sure it
wasn't ich, so it is better to be cautious and do the full treatment than take a chance on a shortcut and lose more fish.
When we are noobs we are better doing everything by the book, and later when we are experts maybe we can skip QT or something because we know what's up.