Does your tank decor have any pockets of trapped water? It could be a breeding ground of bacteria. What type of plants are in there? Plastic? Maybe the plastic is leeching some toxins?
I have a spotted gar, lima cat and 5 pbass and a tsn all smaller than the aro. I have live amazon sword plants. I just did my weekly water change after a week of melafix and now put in aquarium salt. So well see how that works.