One of the toughest lessons that I had to learn the hard way is the importance of quarantining new fish before adding them to your set-up. When I was starting out in this hobby I did not use a quarantine tank and I was lucky for a quite a few years. But then my luck ran out and I had a whole tank wiped out by adding one little fish that was carrying a disease but did not show any symptoms. That was when I started learning how to quarantine fish and actuating following a quarantine process. Based on what I have read, having a tank’s complete stocking wiped out by a disease introduced from a new fish has driven more people out of the hobby than anything else. The good news is this is completely avoidable when you follow a quarantine process.
The below thread in our forum also highlights the importance if you still think it could not happen to you.