How many Koi fish can you keep in 85 gallon tank?

Koi are a type of carp and can get very large. 135 gallons is not enough room for Koi to be kept for a long period of time. They should be kept outdoors in over 500 gallons, constituting a pond. Only fry can be kept in an indoor aquarium but then they will have trouble adapting to an outdoor environment.