Poor guy...I had a very random and devastating betta illness with my last one, Kabuki. He was fine on Friday, I left for the weekend and left my brother in charge of feeding him his four pellets once a day...came back to a vase (it was the 1+ gal size that had always done well for me) that had turned cloudy, viscus and Kabuki's fins were all frayed. I did a complete water change but he still died. And even though I scrubbed out that thing with boiling water, bleach, more boiling water and dried it in the sun, the next betta I tried in it had the same thing happen. I threw out the vase and haven't gotten a betta since. Still no idea what it was or why it happened.
Hope Bellpepper makes it!