Why is he hiding his food all the time and not eating?

I bought my jack demspey about a week ago, I had two oscars before, one around 7 - 8 inches and one like 5 -6 inches, the jack is maybe about 6-7 inches… I have seen him only eat one pellet, other times I didnt watch the fish eat everything because I did notice he will come out every so often if he thinks im not around, im wondering if maybe he is still getting used to his new home? I don’t know, but I did put in 15 feeder fish (minnows) and he didnt come out to chase his food at all! The oscars took care of all of them. I just don’t know what esle to do, I added in some crushed coral to put the ph at 7.5 and I also added some salt to help with the water conditions, and I keep the temp at about 77-78 constantly. The ammonia levels were high but I added something to my filter to help with that and I cleaned it out today, its gradually going down but im not even sure if that was the problem, does anyone know why the jack is hiding all the time and not eating? If you had a similiar problem did anything help in fixing this? Please let me know what you did to help change his habits, maybe adding another jack dempsey? I have a 55 gallon now and I dont want to over crowd my tank but if adding another one to give him a friend will help I will, just lemme know what or if there is anything I can do, thx