In addition the fish should be held in a salt bath as above to aid it osmotically and reduce stress. Epidemics of ulcers will need treatment using antibiotics, either by injection or via the food. These are a problem since they are becoming resistant to treatment. Formalin baths can be used but these are stressful. Certain species such as orfe are highly susceptible to their toxic effects and should not be treated.
Organophosphate dips are to be strongly discouraged since their use is likely to cause toxicity.
Many disinfectants can be used but most carry risks of toxicity if not rinsed out of the system. 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.