The tank was 3g, I know that tank since my cousin had one of those. Usually that tank comes with water conditioner with the right amount, put that much in it could be possible you put to much Nutrafin Aqua Plus in. Most people when they get new tanks they rush and do not allow the tank to cycle, stress the fish out or make mistakes. Did you have a light on when introducing the new fish, did you open the bag a bit so the fish can have oxygen and the new water is let in so it doesn't shock them.
Be careful, and Neons need a heater and Neon can shock easily, my mom use to have a fish tank with these cat fish type of sharks and neons and a lot of other fish and they lived so long since I was little and they died last year since our marble tile cracked and the tank dropped.