You should not feed as temperatures are falling towards this region. Protect the pond using frame with polythene cover. Do not bang on the ice to break it. Melt it with boiling water. Use pond heaters to keep an open section.
Salt also tends to inhibit bacteria and parasites.
Bring them in and treat them. Supplementation with ACE-HIGH is well worthwhile. A carrier state exists in recovered fish, which when stressed can release infectious organisms into the water. These can then attack healthy fish in the same body of water, particularly those under stress or with any skin damage eg. The causal organism is shed from carriers at times of stress such as spawning, handling, high temperatures, and sudden changes in environmental conditions.