Arapaima are very challenging when they are smaller like yours. I fed mine blood worms until it was over 10 inches. They need to be fed multiple times a day too. I fed mine 4 times a day. I would try multiple types of food and maybe see what they were being fed at the place you bought it. Once the fish is larger you can train it to eat the food of your choice. Feeders are risky, but if that is all you can get it to eat, keep some in there since you know the pima is eating them.