The pellets are fine every once in a while but it is better to feed them silversides, beefheart, frozen prawn, krill and other meaty items. Also through some live food in there every once in a while for stimulation. It is ok to feed them pellets amongst that but try a carnivore diet of some sort. You have to understand that koi are omnivores in nature but there main food staple needs to consist of things like peas, lettuce, and watermelon. So dont you think that the koi pellets would not be as healthy as a carnivore diet? I mean like i said it is not neccassarly bad for them but they need that carnivore side as well. Maybe mix them in atleast. Also another note on the size of tank. A clown knife does not turn well, kinda a stiff body and they like keeping there bodies straight. So it needs room to turn around, which means 36" is the best width for a tank.