The reason you are having ammonia problems is because you are overstocked. Koi should never be kept in a pond that small. they produce a lot of waste and need a lot of room. Koi should have a minimum of 250 to 500 gallons EACH with a minimum pond size of 1000 gallons. You really should consider finding a new home for the koi and get goldfish instead.

Filtration,filtration,filtration , sounds like your biofilter isn,t handling the waste output of your fish , is your pond clean " no sludge" or rotting vegetation ? this adds to the toxicity of the water , i believe the formula is 1 inch of fish for every 1 cubic foot of pond surface .
example: if you have a 10x10 pond you have 100 cubic foot of pond surface , thats equivalant to 10- 10inch fish provided you have adequate filtration . and i go by half the recommendation of the manufacturer , if they say your filter is good for 1000 gals. i would subtract that by half if you have fish in the water