Large amounts of organic matter left in a pond over the winter will decay and in spring when the water warms up it will cause a bacterial bloom, this may kill fish directly or may stress them and lead to secondary infections or upsurges of parasites.
Koi become fairly dormant at about 4-5oC (~40oF). You should not feed as temperatures are falling towards this region. Koi do not feed much below 10oC (50oF). Above 12oC they can be fed two or three times a day.
Even if you don't use it as a routine (which you should), use ACE–HIGH (to enhance levels of "stress alleviating" vitamins) in the Autumn.
Sudden temperature changes can be lethal so provide a deep area (5-6ft) for the fish to move into if possible. Protect the pond using frame with polythene cover, if the pond is raised consider insulating the sides. Do not bang on the ice to break it. Melt it with boiling water. Use pond heaters to keep an open section.
The addition of 0.5 oz per gallon of salt into the pool over the winter acts to reduce the osmotic stress on fish, although some plants may not enjoy it! Under normal conditions water leaks into fish along an "osmotic gradient". By adding salt (sea salt or PVD salt) to the water at something approaching the concentration within the fish, this gradient is reduced. Salt also tends to inhibit bacteria and parasites. The downside is that it inhibits freezing and so allows water to supercool in very cold conditions.
Change 25-50% of the water in spring, from the bottom, to remove mulm,
Use ACE-High in the Spring to top-up levels of vitamins that have been depleted over the winter.
Do not overwinter diseased fish with ulcers outdoors, or the bacteria will start up before the fish's immune system does. Bring them in and treat them.
Fish with carp pox are best isolated if badly affected and treated occasionally with CHLORAMINE T to keep secondary bacterial infections at bay. Recovery from this viral disease is usually spontaneous as the immune system gets going by June/July. Supplementation with ACE-HIGH is well worthwhile.
Routine use of a course of CHLORAMINE-T to keep parasitic protozoa at bay until fish can handle them is well worthwhile. At low temperatures the fish's immune system becomes very sluggish, if there are high levels of parasites they can overwhelm the fish before its immune system has "switched on" for the summer.