ok first of all the water condition for the front is horrible mix with the condition for the discus. secondly the front gets too big to be house with discus, thirdly i saw you have a catfish in there. most cat are nocturnal therefor will stress the discus out. fourthly the tank is too small for all the discus let alone discus+front+catfish.
you said youve kept tangs before yet your making the biggest mistake when it comes to fish keeping....
on top of all that i dont know why your waterline is low first thought is that youre neglecting the tank.
edit: on top of all that you have what seems to be 2 oscars in the tank? ARE YOU KIDDING ME???? this isnt a strange mix this is simply just a poor attempt to throw any fish into a tank.