i received 4 koi from a friend of mine, about 7 inches, last week. for the past 4 days one of the koi( a solid white one) lays on its side and on the bottom of my pond. i thought it was dead until i poke it and it swims away. it acts like a dying koi but can not die. All my other fish are fine, they swim up to me for food. have yet to see the white zombie koi eat. any ideas of what is happening?