Well i came back from my mates house yesterday and the really lively one and she eats a lot was having trouble breathing so i dropped in a oxygen tablet and then later i saw that one of it's gills wasn't moving and there was a big streak of blood around the outside, later on i fed it and it's gill started moving a little bit but now in the morning i saw he was hiding and the it's gill wasn't moving again
put in goldfish on your search , takes you to sites which will help you to understand what i wrong with your goldfish.