Awwww...sorry to hear about your betta. That is the risk that we run when we try to rescue little bettas from their sad tiny cups at the
LFS. A lot of times they are just not healthy enough to last, even if we do try to give them a better life. You did a good thing by adopting him though and I hope that you will replace him with another betta in need of a good home.
On a happier note, Sebastian is gorgeous! Even though he wouldn't let you get a good pic, I can tell he is a beautiful fish. Sky is really pretty too. Congrats on your new fishies!