clown knifes grow quick i would just put them in the big tank now might as well little fish can go in a big tank they will grow faster that way anyways thats the best way in my mind silvers will get along with the clowns but b warned though clowns can get pretty mean thats why i had mine grow up with all the other fish and never fed him live food of course i dont like live feeders except crickets that i breed in my basement