I purchased a new betta at petsmart less than 12 hours ago (around 5 or 6pm EST) and put him in a filtered 2.5 gallon tank (I was planning to move him up to a 5 gallon later, but the 2.5 is what I had on hand).
No heater, but it's generally warm here so room temp water has always been perfect for my bettas in the past. I conditioned the water with Aqueon Betta Bowl Plus and fed him the recommended amount of Top Fin betta bits.
For hours he was swimming around his tank happily with much more energy than any fish I'd kept before and he was also the healthiest looking I'd ever kept.
Then about two hours ago he stopped swimming around and was very still. I thought he was only resting but then I noticed that his color had gotten lighter. I kept an eye on him and he just kept getting worse until he finally died.
I don't understand what happened. It was so sudden. I'd like to try again since I have the tank set up again, but I don't want to end up with anther dead betta.