I don't think that's big enough for clown knifes at full size so if you want to keep him for life you need to upgrade, also the reason your clown knife isn't eating is because it is still unfamiliar with its new home. It will need some time to adjust to everything so make sure there are lots of hiding places so he doesn't feel as exposed and just give him time. I have found that staying near the tank for a while, quietly, also helps the fish become acclimated to the environment and you and it allows you to keep a close eye on him to make sure he isn't being bullied, which I doubt it due to the size of all the fish, and to make sure he is swimming fine and doesn't have any injury from transportation. Also, if he doesn't eat the food you put in take it out within 2-3 minutes so it doesn't rot and to make sure that he isn't just checking it out from afar. Be sure to keep up on 10-15% weekly water changes even if he isn't eating and vacuum the gravel to get rid of any excess waste. Also be sure to do plenty of research on you fish so you know what to expect, how big they get, preferred water temperature, pH and hardness, and what kind of environment they live in in the wild.
Welcome to MFK.