My Fish Just Got Sick and Died in 4 HOURS. WHY?

Question
Hello Chris,
     I have had my betta for 4 months in a one gallon tank (no filter). Today, I cleaned his tank out thoroughly since he was going home with my roommate for Christmas. When he was in his temporary tank, he was perfectly fine. As soon as he touched the cleaned tank,he instantly got sick.
      At first, we thought he was just staring at things, since he does that alot. But once his response time to our interaction began to decrease we got worried. We were sure he was sick when he was trying to swim and sank directly to the bottom of the tank.
       We thought he was cold, so we put him under a light to warm him up. He was covered in white fuzz, and was losing color rapidly. I did research online and found this site, where you told someone about saprolegnia and how to treat it. At the same time that I found that solution, my fish died.

How is it possible for a fish to move into a perfectly clean tank and instantly get sick, let alone go from being active to dead in 4 hours?

Thank you for your help,
Mike

Answer
Hi Michael;

I'm so sorry you lost him. That's a shame. It is very possible he was already starting to get sick before you changed his water. That ugly "fungus" is actually a very fast developing bacterial infection called Columnaris. It can be hard to see at first and really can overtake a fish within hours, especially in warmer water. If it was on him already before you could see it, that's probably what happened to him. Here is an article about it;

http://www.petfish.net/articles/Bettas/columnaris.php

At Your Service;
Chris Robbins