These are proably anchor worms and often they can be first seen with the naked eye, one of only two parasites that are, the other is the Fish Louse, or Lernia, these Anchor Worms look like small cotton threads about 1/4" long and white hanging from the body of the fish as well as being actually under the scales of the fish. 

They're treated with Dimilin. 

Dimilin clears the Anchor worm in only a few days. Dimilin is available from many sources, including this one and is applied to the actual pond water at the dosage rates recommended. Please be aware that if one of your fish in the pond has Anchor Worm then there is every chance that many of your other fish will also have Anchor Worm as this is readily transferable.