Sorry to hear about your loss. ): I know how it feels to lose a betta so quickly.
I agree with Pitluvs, 0.5 gallon tanks are great to have on hand for hospital tanks. Sorry I forgot to mention it earlier; most new owners only have the large permanent tank and the cup they came. A cup can be fine to use, but a 0.5 gallon is much better. (:
I also agree that API Stresscoat can be used as a regular water conditioner. No harm in that.
Glad to know that you acquired another betta. I'm sure he'll be very happy in his new home. (: