EMERGENCY Koi dying at fast rate

Hi there. Recently I have bought some small koi and set them up to live in an aquarium until they get bigger. Five fish have died in the course of two days. The first two died and I changed the water, bought a new filter and retested the water before putting in replacements. Now three more have died and I am down to one left. It is a ghost koi and very important to me. I do not know what is wrong because my water is almost perfect the pH is 7.6, ppm alkalinity is 180, ppm hardness 50, 0 nitrate, 0 nitrite. There is nothing as far as I can see that could be wrong. I have been reading sickness and parasite koi pages since the first two died. I have even examined the dead fish and found nothing wrong. Is there anything that you can think of that could be the cause? Also, all the fish would swim around normally and then they would begin to rest on the bottom frequently, then they would lay on their sides gasping for air. They did not hang around the filter at all and they didn't gasp for breath at the top of water.

I don't have an ammonia test, but it's fresh well water.

No ammonia test. 10 gal tank I'm using a tetra power filter for 10-20 gal with added aeration. I just set the tank up no more than three days ago, it doesn't even have gravel.

H3D said:

They are most likely dieing from ammonia poisoning.

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no no. We use the same water for our outside pond goldfish and they are fine. I know we don't have a problem with ammonia.