Craig,
Generally when we see all the fish in a tank wasting away it can mean something serious. Cryptobia comes to mind. One thing of note is your dosing of metro was most likely way too low. 1/4 teaspoon per 55 gal... usually its dosed at least at 500 mgs per 10 gals.. 750 mg is also used. 1/4 teaspoon os a pretty inaccurrate way to measure but it generally is only a few hundred mgs.
I would seriously pick one fish and send it to a lab for analysis....one of the NADA speakers..Dr.Smith has a keen interest in discus and cryptobia.
or you could try redosing metro at a higher dose... since you low dosed already I would use 750 mg /10 gal for 10 days..
My first suggestion of a lab is what I would personally do.
hth,
Al