Question
I have had a goldfish for 6 years and it is now eating the tail of a fantail that I have had for five years, both have been kept in the same tank.

Answer
Goldfish are known to pester or nibble on each other.  If your fantail is female and the other one male, then it could be that he's trying to eat the eggs out of her and the tail is getting eaten too.  Or, if any goldfish has an injury or lethargy, other goldfish may take advantage of that and eat parts of the sick goldfish.  If you can separate the fish, that would be best.  If kept together, the tail damage will only get worse.  To treat the injured goldfish, I suggest aquarium salt (a tablespoon per 3 gallons), antibiotics like Maracyn by Mardel, and MelaFix by Aquarium Pharmaceuticals which is an all natural antibiotic and aids healing.  The tail probably won't grow back to how it was but it may grow back some.  Good luck!

Robyn
fishpondinfo.com