You added too many new fish too fast. Fish poop gives off ammonia and it can kill the fish. Your tank has good bacteria that takes care of the ammonia, but when you add too many new fish the good bacteria can't handle all the new ammonia. I know you already took action, but 20% water changes will reduce the ammonia until the good bacteria has a chance to grow.

Also, “feeder” fish carry a lot of disease. You should quarantine the new fish for about a month. Look for ick (white spots), or anything out of the ordinary with the new fish.

Since it seems like a healthy tank went bad so fast, I'd bet it was the ammonia.