It would be just a guess and you may get other ideas, but my first thought was the algae killer. It has been known to deplete oxygen levels in the water, causing fish to suffocate.

That was my first guess also
Anne

I'm putting my bets on Algae killer. I don't think I would want anything with the purpose of "killing" anything in my pond. Algae Fix is noted for many kill-offs.

It's better to take the slower and natural route when dealing with nature. Nature is your friend, chemicals are not.

this was the second application, why wouldn't the first time have done anything? i only used about a tablespoon too, according to the directions. its says safe for fish and other plants, just not algae. i don't have the bottle anymore, but it's sold at meijer. white bottle, green and black label.

Because you added more of the chemical algae killer and the content (PPM) in the water went up!
"Horton"

Cody, I should have added.
If you are going to take a chance on using those additives to control algae or anything else, you must aerate the water vigorously, to keep the oxygen content up.

Before you purchase or think about adding any of those chemical products to your pond, for whatever reason, it would be wise to check here for opinions on the product and the experiences other people had with them.

You have to be careful Cody, there are lots of charlatan "cures/fixes" out there that can possibly do more harm than good.
"Horton"

the water is very vigorously aerated. like i said i have a pump and fountain going all the time. the other fish are all fine.

I withheld from saying this in my first post because I thought others would chime in. I know you have very few fish, but you also have VERY little water. You have one koi and one koi needs 500 gallons, you have a max. guesstimate of 250 gallons. You also have other fish. IMO you are already over stocked and you're adding chemicals. Your pond had not had the time to develop natural benefical bacteria and I would think what beneficial bacteria that did happen to develop, you unknowlingly killed. Slow it down and let nature take it course. As for the koi, I feel sorry for it. It needs more space. Sorry for ragging on you, but you're wanting spristine results in a poorly planned enviorment. Read a bit more about ponds and fish, etc, etc. You'll get the hang of it and hopefully not at a lot of more fish deaths.

thanks for the concern but i did do research, for about 5 months before i even considered putting in the pond. i also know that the smaller the pond the faster the algae will grow and i was trying to make sure it didn't get out of control. as for the koi, i know it'll be a problem, but he has a permanent home in a 2000 gallon tank eventually. it's only about 2 inches long, so it should be fine for right now and the other fish were all less than an inch.

Hi Cody. I have a little pond too 500-600 gals. I am in west Mi north of Muskegon. Where are you? I would worry about overwintering your fish. My pond is barley big enough for my goldfish to over winter in. What are your winter plans?

that is a very good question. i have several friends with ponds but they're all bigger than mine. i've been told that getting a water heater, like for a horse trough. works. another person told me that she puts down a layer of plastic and then a layer of straw and another layer of plastic. my plan was to put the goldfish into my fifty gallon aquarium and the koi into a stock tank along with the two tropical waterlilies i got. mostly so i can observe the pond and see what happens. i'm sure other people here, will know so much better than i do.