Why did my goldfish die?

I added 2 little goldfish to my tank already having 4 goldfishes, they were lively wen i bought them but one of my fish started to act different(while other one was lively) n died today. Now the other one too is spending most of its time on the floor n not swimming much. My Ammonia,nitrite n nitrate r all fine as i did the tests.My other 4 goldfish seems to be happy and lively though.I also followed all the procedures while adding those 2 goldfishes to my tank.I do partial water changes of 20%- 25% once or twice every week. Can anyone tell me wat happened?