My goldfish are eating eachother ?

I noticed some of my fish were disappearing and the fish didn't die I know it was eaten. Exactly the day before it was perfectly fine and was healthier then the other fish. One fish I had, I thought it was going to die because it was laying on close to the pebbles on the floor. But now it got fat, very fat. I also feed my fish everyday and had them for 6 years one for 2 years and the one that died I had 2 years too. Please no sarcastic/rude answers.