Koi did it, Fish in that new of a pond are going to put off waste, bad news, it's only going to get worse with Koi. They are NOT good start up fish at all, produce way to much waste, are not new water hardy, and stay hungry and grow big. At this point the best things you can do are,
List of needed pond supplies:
2. Bacteria supplement or Bacteria booster-
Recommend- Pond care powerpacks- least expensive, drop in pouch. I've tried them and the work!
A GOOD drop type test kit. Don't waste your money on strips, buy one that you have to use the drops to measure and learn how to correctly use it. If your going to even attempt keeping koi, this is a must IMO
Recommend Aquarium Pharmaceuticals AP master test kit, Again for a newbie this is not the best but works good.
3. Get a big bottle of Chloramine remover, Declor. take the guess work out of what you have in your water, Chlorine or Chloramine, if you treat for the second one you cover them both.
Recommend Tetra Pond Aqua safe, any work there are better ones on the market but this is inexpensive treats more gallons per oz than most and works.
4 A net, Trust me your going to need it
Recommend the largest you think you can move around in the pond! Don't waste your money on a 4-6" net, At minimum 12" prefer way larger! HD sells a decent Koi net that is about $20 it has a retractable handle, nice size opening and affordable, works great for fish under 15" That should be good for you for now.
The other products are all available at PetSmart, I use them because they are all over the US. Other stores may have different product lines, some better some not, some expensive some cheap. My recommendations are for cost, quality, and proven they work. I have used them personally.
Be aware more may post and add to the list!
What not to do!!!!!! If you change any water, no more than 10% per week unless instructed by someone for an REAL emergency.
Do not add chemicals your not 100% unsure of, If uncertain ask and get good advice, better to hear from two, three or four people than a sales person!
Do not tear down your filter, leave it alone!
DO NOT OVER FEED!!!! over 90% of ALL fish deaths are due to feeding and water conditions! This is a fact! You feed they poop water goes bad, THEN fungus's parasites, and such attack, they can happen at any time but are most common when too much food and bad water contribute. So don't medicate either, get good information, know exactly what your treating and if at all possible treat in a separate tank or container not the pond. I'll stop there, others may add, Have patience, learn about what is going on and what you want for a end result. BTW, I'm warning you, Koi get big, they get addicting, the get personal like a dog, they require Vet assistance at times. They may be kind of cheap to buy, They are NOT cheap to keep!