Why did my Koi fish die?

I bought 7 Koi fish three days ago from an upmarket pet shop. I placed them in a pond in my garden filled with fresh tap water. I've also been feeding them the prescribed Koi fish pellets. When I came home this afternoon one of them was in the throes of death. It was swollen to almost twice its size and was... show more I bought 7 Koi fish three days ago from an upmarket pet shop. I placed them in a pond in my garden filled with fresh tap water. I've also been feeding them the prescribed Koi fish pellets. When I came home this afternoon one of them was in the throes of death. It was swollen to almost twice its size and was kind of twitching in the water like it was having a seizure. It died like one minute later. Did it have some sort of disease and could it have infected the other fish? Could it have been the tap water? Our tap water is relatively clean, but obviously there is chloride in tap water so could it have been that?
Update: Honestly, I bought the fish on a whim, I asked the lady at the pet shop how one needs to take care of Koi and she didn't mention anything about cycles and water conditioners. I literally just placed the fish in water and gave them food. Another one has died since I wrote this question. I do take responsibility... show more Honestly, I bought the fish on a whim, I asked the lady at the pet shop how one needs to take care of Koi and she didn't mention anything about cycles and water conditioners. I literally just placed the fish in water and gave them food. Another one has died since I wrote this question. I do take responsibility for not doing my research beforehand, but I'm also pretty pissed off at the pet shop for not giving me adequate info after I explicitly asked them about caring for Koi. I'm seeing a vet tomorrow so hopefully he'll be able to sell me a water conditioner and tell me how one cycles water. I'm also installing a little waterfall so hopefully that will increase the oxygen in the pond. I think my fish are doomed though. I have none of these things you are all mentioning- cycling the water/conditioner/filters/a very deep pond etc. I'm considering just giving the fish to the vet, At least then they won't all die.
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