@Amber , we're going to need a lot more information in order to help you. I'm assuming (and we all know what happens when you assume..) that the koi is in an indoor tank since you posted in the "indoor tank" forum. Koi don't typically do well in indoor tanks simply because they are not large enough for the fish. How large is your tank/pond AND how many fish do you have in it already? Do you have the results of a recent water test (preferably from a liquid/drop based test) like the "API freshwater master test kit"? Please post the numbers and not just your interpretation of the results because we can often misinterpret them or miss something important or vital without the actual numbers. Other members will chime in soon and may have other questions for you, but these few should help get the ball rolling.
Hello and welcome, @Amber. As @ShawnInfirmity said, koi aren’t really good fish for indoor tanks......or any small body of water. Unlike tropical fish, koi will grow very large and do so very quickly, so that 5” little cutie will outgrow your tank very quickly. As fish go, koi produce a LOT of waste. In general and ideally, you should start with 1000 gallons — and that’s just for one fish.
Not to sound rude, but maybe koi in a fish tank are not the best choice for someone just starting out, not knowing anything about the hobby,
What kind of filtration do you have? Yes, PLEASE get an API liquid pond test kit (not strips) and test your water right away for ammonia, nitrites, and pH. Give us the numerical results not “normal” or “OK.” We will try to help you, but I fear that these fish need to be re-homed, in their best interest.
@Amber - welcome to the GPF. Sorry that it's fish trouble that brings you here. It could be a water quality issue that is affecting your fish, or it could be some kind of parasite or disease. Could you post some pictures of your fish?
As for the bigger issue, I would say return the fish where you got it, but obviously they aren't vetting owners, so maybe you can find someone with a pond who could give your koi an appropriate environment in which to grow and thrive. Your five inch koi will be 12 inches long in the blink of an eye. And then 20. And possibly 30 or more before it's done. Sorry to be the bearer of bad news...
My "pet" peeve (haha! sorry, couldn't resist!) - when you walk into a pet store and see koi being sold in aquariums with no indication that these are not aquarium fish. Our local pet store has an outside tank set up for koi with a sign indicating that these are pond fish and they will not sell you koi without seeing evidence that you have a pond. Selling them to people who are not aware of what they are buying is irresponsible, in my opinion.