If your tank is not capable of supporting the mandarin with live food, you have a ton of work ahead of you. They have terrible eyesight apparently. The food must be right in front of the fish, also if the food isn't wafting around in the water, just above the rocks it will be ignored. Mandarins will not eat from the water column, they will only eat food that is on the rocks, or moving just above the rocks. Don't bother chopping the worms, its not necessary, my little guy would slurp down a full blood worm like a giant spaghetti that was nearly the same length as him. Hikari sells a very small mysis in cubes, if you mix a few with lots of water, you will likely get it to eat those as well. Plan on putting LOTS of wasted food into the water. Hopefully you don't have too many other hungry fish that will go after his food.