So sorry to hear that.
I haven't used that particular water conditioner but it seem like an awful lot for that small amount of water. Are you sure the dose is for .5 gals and not 5 gallons?
And I would do a complete scrub down of the tank and gravel or rocks before you put another betta in there just in case there is something ligering from a possible sick fish.
And yes, test your water after you let the new set up run for a day or two before adding anything.