Your fish might be suffering from shock. Do a little research on the fish see what there personalities are and what they eat. Eventaully they should eat whatever your giving them. If you do want to give them some algae, you could either buy an algae supplement or you place a rock from their tank and put it in a water-filled container. Place the conntainer in an area where it can recieve direct sunlight. Algae should start to grow on the rock. When its covered, take the rock out and place it in the tank for the fish to eat off. Put a new rock in and repeat. That should help!