See that's the think. My last betta ended up having cottonmouth or something and I was told to float him in a container. I used one that had been washed with soap, and he ended up dying pretty quickly. That's why now I only use things that have not been in the kitchen lol. I was also told to double the amount of AQ salt after a day or two... That didn't work out so well.
If I rinse another container really well, would the big walmart sized betta cup be okay for one? I don't want it to be too small and end up having an ammonia problem... even though I would be changing the water every day.