discus question

[emster1]

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

[Rave]

How long have you had the aggressive discus, in my experiences, a new discus in the aquarium can often change the pecking order. leading to one or more showing the behavior you have described.

[emster1]

they have all been in the tank together for 6 months

[emster1]

the aggressive territorial and not eating has just started today

[MuckyFish]

Sounds like he is stressed,
Could be sickness if not eating perhaps.
Any change in coloration?
Any changes with faeces?

Do a water change.
I'd consider foods that medicate for parasites on the off chance they will eat.

Setup your quarantine tank, whether it's medical or social issues QT tank running is a useful resource.

[emster1]

could i add some aquarium salts? i did water changes every other day, and still acting "distant". I dont see any change in coloration or feces, no signs of external trauma or spots.