Welcome to SD .
I think your 0 nitrate is wrong ..Usually people get zero from not doing the test properly .
Are you shaking the living daylights out of the #2 bottle when testing ..Sometimes it needs to be hit on a hard surface as the chemicals in the reagent settle to bottom of bottle and get hard thus skewing the test results ..
0 nitrates means an uncycled tank but yours has been up long enough to be cycled and with the fish you have even if you did daily water changes you would likely have 0.
So water quality first might be an issue.
Next what are your water parameters [pH /GH /KH ]and why are you adding the discus buffer?
Those things usually don't work well and just cause pH swings that are worse for the fish then just leaving it the way it comes from tap..
What is the ghost fish ? A ghost knife ?
Has the tail been split for long ?