My goldfish have got finrot, tailrot

Hi
I've noticed recently that a couple of my fish (goldfish) have lost
their tails. I'm assuming this is tailrot\finrot. I've googled the
subject and discovered that it's quite easy to treat by medicine and a
partial water change but is there anything else I can do to make sure
it doesn't happen again? My pond is an outside manmade pool which
contains about 40 fish (4ft x 3ft x 4ft) which is about 10 years old.
I tend to leave the pond alone to develope as a natural habitat, so no
feeding. They live on the plants that naturally occur in the pond and
til now they seemed quite happy.
Thanks for any suggestions.