I don't recall what you have for filtration. If you have a good filter, it should only take a few weeks to get your bacterial population reestablished. In the meantime you should get your water tested regularly. Unless you love traveling to the fish store, it would help to have a test kit. While you are waiting for the pond to return to health, there are some things you can do to help your fish.
Change water. When you do a 50% water change, you remove 50% of the chemicals dissolved in it. That's enough to reduce "deadly" levels of ammonia to "toxic" levels. If you get the numbers from the tests, we can tell you how much water you have to change to make the water healthy for your fish. If it were my pond, I would do a 100% water change to start with and then test to see how much water I would have to change to keep the ammonia as close to zero as possible. It's only for a few of weeks. You don't help your bacterial population by keeping old water. The bacteria you want live on the surfaces of the pond and the filter not floating in the water.
Reduce ammonia production. Fish release ammonia. This is a breakdown product of the protein they eat. There are two ways to lower the ammonia production. One is to have fewer fish. (I know, this came too late.) The other is to feed less food. I recommend feeding very lightly every other day. By very lightly, I mean just enough that every fish gets a pellet. Don't worry about your fish starving. They can go for a week with no food whatsoever without harm. If it really bothers you that you aren't feeding, you can cook some chopped spinach or other leafy green veggies and feed that to them. It's like eating lots of greens when you are on a diet.
When people told you how to tell goldfish from koi of the same size, they didn't say much about the most important difference. Koi get to be about 3 times the length of goldfish and about 10 times the weight. Your pond is only about 200 gallons, which is fine for a half dozen goldfish, but too small for even one koi. You will have to find new homes for your koi as they grow. For me, it would be easier to rehome them now, rather than after I got attached to them.
