As Stanzzzz7 said there are many reasons for flashing.
The bad case of ick in my photo was brought on by not quarantining some fish long enough, I only did 3 weeks, should have done much longer, would have saved me a lot of grief.
I treated with 3ppt (parts per thousand) salt for a month, and no fish died except the new fish that brought the parasite in (Not the ones in the photo).
But I was adding hand fulls of rock salt with every water change to 300 gallons of tank and sump, and vacuuming heavily to remove dormant ick cysts.