Last month, my one and only betta Leftenant died. ;__;
I got him at the beginning of the school year for an assignment in biology, and he started out in a half-filled soda bottle and worked his way up to a heated, filtered, 5-gallon tank. One morning he didn't want to eat, and that night it was over.
I did a ton of research prior to getting him, but not enough - I didn't know about tank cycling, and postmortem showed his gills had been burnt up because of it.
It's pretty depressing that it's the first thing I'll actually get done here, but it's only right.
Rest in peace, Leftenant (Lieutenant, LT). Hope you found better waters. If nothing else, your death has ensured a better life for other betta, not yet passed under the Rainbow Bridge.