Sick goldfish with velvet (pic)?

try boiling your gravel to clean it properly this does work and any imperfections come out also get another fish goldfish hate to be alone they are not solitary animals this is why so many people lose fish because they get lonely. maybe you have done something to upset the balance of the tank do you treat the water for lime and so on , when i do a water change i only have goldfish and i just use normal tapwater without any additives, one of my fish is 15 years old, so don't add anything to the tank just set it up as you normally have but leave the tank and the filter running empty just water this will take anything abnormal out of the water leave for about 1 hour.