Help Me Please

Hi there im new on flwoerhorn and i just got my flowerhorn, he's 3inches long an dim havign problem with him, i got a 15 galons tank...For few days that my flowehorn had been adjusting, i got this problem on his food, i feed him pellets, but the problem was, he chews the pellets then spits it out and does it 2-3 times until he gaves up on it, i dont think the problem is about the size of my pellet cause my brother's FH could easily swallow it which is much smaller than mine...for the past few days that he had been eating up, came up this day that he got no appetite, he follows my hand but when i throw over the pellets, he disregards it... and my FH has long white poop coming out...some said its probably hexa infection, ive already done water change 80%,cleanse my filters and added 500mg metronidazole 1tab to it,,,but i got no heater so i just put some warm water in it...anymore problems my fish have? have i done something wrong?how long hes gona be ill? cause he'ss like staying in one corner.