I have a 2000 gallon pond, and all has been well for about 2 years.
Recently I added 6 new small koi after quarantining them for 2 weeks and all
looked well. Shortly after introducing them to the pond I noticed that 4
of them were "flashing" and have noted previously that people attribute this
to flukes. Since then the fish seem to just swim around in circles near the
surface with all fins kept close to the body and, in a couple of cases, it
seems that a few scales seem to protrude slightly from the body. These fish
have all gone on to die. I have no idea what flukes are or how to treat
this, if indeed this is the problem, and any help would be gratefully
appreciated.

TIA

John