When I was 15 years old I had a red belly pirhana tank. 3 of them in a 40 gallon. It was my pride and joy.
Unfortunately, I knew nothing. The tank was great for almost a year, until I decided I was going to rearrange the tank and do a monster water change. I think it was the combo of stirring up the substrate and adding 20 gallons of uncycled water, and the tank did not have enough beneficial bacteria left to fight off the ammonia.
Strangely, it was easier to get over this because I learned what I did wrong. It's when you dont KNOW why it happened that must drive you nuts.
Anyway, whatever happened, hopefully you learned something, and anything you can share with us will serve as good knowledge for us too.