What should I know about winter feeding of Koi?
Fish metabolism starts to decline below a water temperature of fifty five degrees, which reduces the requirement for feed. However, "feeding fish in the cold" is not inherently DANGEROUS until the unique situation arises in which the fish are fed, then severely chilled  (e.g. plunges to the high thirties or worse) at which time the lining of the GI Tract may be severely damaged.
If you're in Southern California and the fish are cold, and freezing weather is unlikely, you can surely feed wheat germ in small quantities as long as plunges in temperature are excluded as possibilities. I am in love with Cheerios as a cold water food. The reason is that the metabolic by product of Cheeriods metabolism is LOWER in Ammonia, and the filter (which is also cold) has less work to do reducing nitrogen.
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