i have a 55 gallon tank that has been cycled for several years. ammonia is o, nitrite is 0, nitrate is always trace. i have 5 4inch discus, two cory cats, and two cardinal tetras. eheim filter, bio wheel 350, and aqua clear hob. i do water change every other or every two days. temp is 87. i feed hikari floating pellets and some flake and occasional frozen blood worms. suddenly this morning one discus is in the bottom corner and has become aggressive if anyone swims near him but he wont eat. i use prime with every water change, and i see nothing on his that looks abnormal. not sure what to do