|My fish is basically energetic and eating, but it's in cold water during winter and it's becoming covered with white flecks.
Fish with the appearance of what you showed me are in serious trouble, but are often helped by warmth, clean water, and Fluke Tabs. Often, these lesions are the result of:
1) Low pH
2) Flukes (very common to appear like this in cold water)
3) Virus (papilloma) as a result of chilling water (These pictures look MOST like Papilloma Virus or also Pox lesions.
4) In some rare cases bacterial infection
When you warm them up in well aerated faciliites and treat them to either Fluke Tabs, SupaVerm or Praziquantel, and ensure a suitable pH and water quality, the fishes' white flecks begin to peel off en masse and many have been saved without caustic remedy. Apparently the #3 cause above is simply ameliorated by little more than warmth, if only during the warmer parts of the year.