The cause of 'Wasting Disease' as it is know, can be attributed to either a heavy Parasitic infection, such as flukes which seem to bother just that single fish ... but this is normally found to be the case in young fish such as those about one year old .. 

The treatment is of  course to treat the pond for that parasite - once that parasite has been identified using a good microscope, with individual treatment, such as a salt bath using up to 1 ounce per gallon with plenty of air diffusion in the bowl being used, - these salt baths should be no longer than 30 minutes and stay with the fish to make sure that it does not stress up... if it shows signs of this stress then return the fish to the main tank...

If however this wasting disease is in a more mature fish then this may in fact be an internal problem with that particular fish - it could be tuberculosis of the liver or some other liver disorder which may have been caused by feeding a poor diet or poor wheat germ based foods and in such cases it may be worth considering veterinary advice.