How long has the tank been set up?
How was it cycled?
The stress of being bullied can allow the fish to become ill. I had mine get parasites. I am guessing reduced the immune system and the little buggers took over.
I used this medicated flake. Sometimes you can get medicated flake locally.
The fish I have had do not seem to like the soaked liquid medication /food and do not
You can check your fishes poop and see if it looks stringy and whitish which would indicate parasites.
Are you using test strips for checking your parameters? Often they are faulty.
If you actually have perfect water in there, you could have some other issue with the fish.