if you see "fuzz" on your fish
ick is not fuzz,
[ick looks like sugar crystals, it is rare in cool water]
but the same medicine may work...
we use, sell, and suggest malachite + formaldehyde...
salt at 5 pounds per 150 gallons will do the same thing (usually) for "fungus"...
true fungus on fish is unusual...
some bacterial colonys look like fungus...
doesn't matter, treatment is the same...
salt at 5 pounds per 150 gallons is harmless for 10 days...
malachite+formaldehyde needs to be repeated daily (or every other day) it goes away by evaporation ...
7 - 9 days max. ...
you might also have something very rare like low (or high) pH burn ...
I've seen fish (not koi) "fuzz" up after a massive pH shock like 9 down to 4...
that is physical damage (same as a human would get by sticking your arm in a bucket of acid)
but this situation is rare (or should be) in ponds or fish tanks.