Right.
Fishkeeping.....fish keep themselves, YOU keep the water, and feed.
So, for disease to set in, needs a trigger, such as stress from bullying, bad water quality etc.
If there is nothing obvious going on, 1st thing to do is quite a big water change, dilute whatever crap they are swimming in.
then treat and visible disease, such as fungus, swimbladder issues, ulcers etc, as stated salt is good for swimbladder issues, (iirc 1 teaspoon per gallon, but been some time since I worked in the trade.)
IF you can seperate the poorly fish, this can be beneficial if it remains in a good enviroment. however treat both normal tank and treatment one, kill off whatever is the problem.
As for plecs, there are many species, and a fair few of them are fine in a wide temperature setting, even ok in ponds during the summer.
adding new fish, which dont last is rarely anyones fault, its stressful moving the fish, and they go from one water quality to another, different temp, ph, hardness etc etc etc, and the shock of this move often upsets fish, hence the 24 /48 hour guarantee's you get, also if you filter system isnt up to scratch, adding new fish increases waste in the tank which the filter takes time to adjust to, which on top of the stress, can easily see off the new fish. (the existing stock have had no stress so cope better)
Hope this helps.