Whats wrong with my Black Moor fish?

My beautiful Black Moor Goldfish can hardly move he just wriggles around on his side very slowly and then sinks back to the bottom. His lovely long tail is all torn and he has little white spots on his body. He is one of my first fish so i need help knowing whats wrong with him and what to do. At first when i saw his tail i thought the other fish had injured him because they used to chase him and bully him around but i know there are lots of illnesses for fish like fin rot e.t.c. Please help because i love him a lot and am constantly worried about him!