Not sure what is wrong with my pet koi?

i have a 5/6 inch koi. I had this fish for many years and now it has been acting very strange recently, i think i got this fish maybe 6 years. the koi lives in the tank, and for about a year it stayed alone in the tank and started to get to big for the tank so we got a bigger tank, so recently we changed the tank and got two other koi's and 4 comets and the koi was fine but a day ago the koi is very inactive, it has its top fin down and swims seldom and lies on its belly on the bottom of the tank it still eats but not so actively i am really worried and i need to know whats wrong with it, your help would be really appreciated