leaky gills

i have a ranchu goldfish, approximately two years old. for the past
few months he has been gasping for air at the water surface. he never
swims further down than two to three inches except to feed. i change
the water regularly, and have a large air pump.

today i noticed air bubbles escaping from his gills when he gulped air
from the surface. is there anything i can do to help him, or is
euthanasia the answer?

thank you in advance.
maggie