check your water parameters, low pH will make them flash.
this is the time of the year when fish get frisky and spawn so the ammonia
could be sky high.
do a partical water change, like 500 gallons.
the best treatment for lice and anchor worm is "anchors away" that contains
dimilin and it stays in the pond doesnt need to be removed, no water
changes. it is best to eyeball a fish up close and see if they actually
have lice. dimilin has an almost non existant toxicity for fish, but it
will kill all the crustatean type critters in the pond. not a good thing
usually.
if you dont have salt in the pond and dont live on a coast (or Utah), then
add 0.9 lb of plain salt crystals (used for softeners with no additives) per
100 gallons of water. this helps them to shed their slime coat and whatever
might be itching them.
ALTERNATIVELY, do NOT add salt, but use some kind of formalin and malachite
green combo and that also does not need to be removed. It inactivates and
blows off over time.
Ingrid


"burky" wrote in message
oups.com...
I have a couple koi that are flashing, which I believe could be fish
lice. My pond is approx. 2000 gals.with waterfall & stream. Could
anybody what would be the best medication? Is there anything that
don't require a big water change?

Thanks for any info