How big is he/she? Mine is about 22 inches long. She eats everything that I throw in the tank. The thing with horns is that their sense of sight is not that great. Mine runs into rocks, the glass, etc. But her sense of smell is unmatched. I have to disagree with the posts that say to feed live food. A horn shark is not a fast moving, stalking animal. Mine eats hunks of meat. Salmon, tuna, shrimp even formula one and brine shrimp plus. As soon as the food hits the gravel, she begins to stir. Her sense of smell guides her to eat and she eats without any problem. The thing I would try is feeding food that sinks to the bottom. Horns tend to spend a lot of time cruising the bottom, in addition to the open. They are not fast striking animals. IMO, live food would be tough. Try chunks of salmon or shrimp. Just make sure that it sinks to the bottom. You might even try to bury it slightly in the sand, leaving the top of it exposed. When my horn gets really excited, she takes in the gravel and all. It is cool to watch it get flushed out through the gills. Mine is alos in a 300 and she eats like a champ. She is not picky at all. I think it is just getting the food in position for the shark to take it. Let us know how you make out and Good Luck. The shark will come around.