so sorry for your loss , i was hoping he still alive. I also do only full water changes for all my tanks. So not sure what could happened. Weird, something get in to the water.
How long you had him ? Are you going to buy another one? I am assuming you don't have another betta since you saying something in the water which would effect another one if you had it.
You can just rinse the tank and everything in the tank with hot hot water and let it air dry for a week or longer to get ready for another betta. I hope you will rescue another one.