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Stop feeding when water temperature drops consistently below 50 degrees F (10 degrees C). Koi metabolism slows dramatically in cold water, and undigested food can cause serious health problems.
In cold climates (zones 3-6), a floating de-icer or pond heater is essential to maintain a gas exchange hole in the ice. Without it, toxic gases can build up under the ice and suffocate fish.
In mild climates, yes. In very cold climates, you may need to shut down and drain external filtration to prevent freeze damage. Keep some form of aeration running to maintain oxygen levels.
A minimum depth of 3-4 feet is recommended. Deeper water (5+ feet) provides a more stable temperature zone at the bottom where koi will rest during winter. Shallower ponds may need a heater.
Begin winterization when water temperatures consistently drop below 60 degrees F, typically in early to mid fall. This gives you time to complete all preparations before the first hard freeze.