I really wish I had joined betta forums before I got a betta/did more research. I spent about two hours in petco deliberating whether or not to buy Fred(I did :3) and while I was choosing food for him, I was also doing research on my phone xD
Because of the wonderful people here, I now have a great thing going! Cycling tank, happy fish, GREAT advice, HEALTHY FISH(thank goodness, he just recovered from some parasites). I also asked my friend's friend about where he got his daphnia, and discovered that there was a pet store nearby that carried live food! :D :D :D
I think it's good to know what you're getting yourself into before you commit to buying a betta--something I wish I had done(being informed...cycling the tank before bringing a betta home).
You didn't kill him, it was the fungus that killed him--not your fault. You tried your best and it just didn't work out..at least you know :) -hug- If you want to try again, try again. But this time be informed about your decision and make sure that you have all the supplies before bringing another one home(making sure the temp stays constant, water conditioner, test kit, food, TIME to change the water, $$ to buy medication if the fish needs it). Fred is in a half gallon right now and he's doing just fine(my 5gal is cycling).
Don't be discouraged because of a mishap--it happens. And all you can do is try again :)