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.