That tank is not suitable for those fish, none of them, even one is small enough to fit in the tank. Jacks get nasty, blue dempseys are on the docile side, the firemouths won't be able to stand up to the JD for long, and the Hemichromis is gonna be nasty as well. You will need a tank at least 125gal for those fish as a minimum.
You still have not provided us with Ammonia, nitrite, and nitrate readings, of the size of the fish.
However, in short, your hemichromis is not eating because he is gettin the crap kicked out of him, especially in a tank that small. He may be eating when you aren't watching as I'm sure he hides in areas where food does settle and is getting something to eat.
Call me mean and what not, but either get a large aquarium, AT LEAST a 75gal FOR NOW, or take the fish back to where you got them.