If your fish died within three hours, it couldn't possibility be ammonia. Ammonia builds up over time in the term of days. No way it could kill fish that quickly. Even if they were low quality weak fish.
How big is a "A big tank"? Please use gallon or liter when you refer to the size of a fish tank. Otherwise you are not giving us any clue of its actual size.
In order to find the cause of fish dying so quickly, we need more information.
Did you use tap water? What aquarium water conditioner did you use?
Did you let their plastic bag sit in the fish tank water to allow the temperature be the same?
Did you add the tank water to the plastic bag little by little to let the fish adapt to your water?
Did you have a filter?
Did you have an air pump?
Perhaps you need to read our article on Why did My Fish Die?
While Walmart indeed sells fairly weak fish in poor conditions, if they die within 3 hours it must be something you have done incorrectly. Otherwise they would all just die in Walmart's tank within 3 hours, right? 
By the way, I do not quite get the part
Quote:The next day, a tank that had twenty gold fish in it was down to two small ones, that died soon after.
Were you referring to a fish tank at Walmart with 20 goldfish in it?
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Why did my fish die in 3 hours? (Fish bought from walmart)
