Woke up this morning to find my betta on the bottom of the tank dead. I'm really upset about it too. Yesterday I noticed that his scales were starting to look a little pale, but I figured that it might just be from stress as I had to move the tank twice in the past week which involved an almost 100% water change both times.
My other fish are okay (three albino corys) so I am also wondering if they might have introduced some sort of bacteria into the ecosystem that might have killed him.
I'm going to take a water sample to the pet store later today too.