Problem with my baby flowerhorn?

my flowerhorn fish is very active and aggressive.its very playful.it bites my hand when i put it into aquarium.but today suddenly my fish has became inactive.the colour from its fins to belly has became very dark red.the shop keeper told to add 2tsp salt and a tablet of teramycin but still my flower horn doesnt come up it just sits down and doesnt eat at all.but before this when i used to fed him food,it used to jump like anything and catch the food and eat it.plse help me.i love my flowerhorn very much.....