Dying Goldfish !!

Question
Hi i was hoping you could help me. I have goldfish they have all been fine until recently. I woke up n found 3 dead in the tank, i removed them and all the other fish were fine but about 2 hours later i found another at the top of the tank hardly moving and its mouth and gills were barely moving either that died aswell and about 30 minutes later i found another barely moving i moved it 2 a separate tank with a bit of salt in it n after about 20-30 mins it was swimming fine. i don't understand why this is happening. please help

Answer
How long has the tank been set up?  How big of a tank is it?  How big are the fish?  how many goldfish total did you have/  Have you tested the ammonia, nitrite and nitrate readings?  Please let me know and I will be able to help you more. Sorry for your loss