Oh I am so sorry, that is very sad. I know my last betta died suddenly too .. and I took very good care of him. I hope that you don't let this keep you from getting another one. They are great fish and loads of fun as you know. Next time when you go to the store to pick a new betta make sure you look him all over for illness. See if you can get him to flare his fins to look for rips or holes in them. A betta with bubbles in the water is a happy betta. I try to pick a betta that is feisty.