If your Koi and other pond fish are not eating .. then this is normally a sign that something is wrong and that the fish are not happy with the water, or something else in the pond ...

The first thing that you should do in such cases where the fish are refusing to eat ... is to check the water parameters ... check for Ammonia and Nitrite first of all .. if there is a high reading on either of these - then this could be the reason that the fish are not eating ..  Check the oxygen levels, which again if those are too low, could cause the fish to go off their food .. 

Check for toxins in the pond water another cause for fish not eating .. and then of course check the water temperature .. fish will not feel like searching, or eating food if the water temperatures are too low - and this can of course be seen during the cold winter months if the pond in question is not heated, or at least covered over to retain whatever heat there is ..

If the problem, is more confined to just a single fish - then catch the fish and place it in a bowl with pond water in the bowl, using a Koi Transfer tube or Sock, to ensure that you do not drop the fish or cause too much stress to the fish, as clearly the fish will already be under stress if it is not eating ..  Examine the fish for damage, or sores especially under the belly ... and if you do find a sore then this may well be the reason ..   treat that wound accordingly and keep your eye on the fish for the next few days ..  If however you do not find a sore or wound, then the problem may be gill congestion and in that case I would suggest that you give the fish a salt bath, making sure that you place at least one air stone in the bowl, and add salt at the rate of 1.5 ounces per gallon and stay with the fish to ensure that the fish does not lay over as Salt is a mild anaesthetic, leave the fish in the solution for approximatley half an hour if possible and then return the fish to the pond... again keeping your eye on the fish for the following few days ..