where did you brought thats discus ?......from what you discuss i know it was infected before you brought the fish..maybe it seen to be ok at the previous owner but it already start internal parasite...water change alots dose not help if didnt apply the suitable and correct dosage amount of madicent....please try as below method as I already test according to andrew soh lesson...
1. remove to a clinical small tank to avoid others infected ( i think others already infected but if they can manage to fight back thats ok, the better is to treat all of them together in one time)...
2. second day - add PP (potassium permanganate) at 2 to 2.2mgs per 100L of water + 150gm of normall cooking salt @100L of water...this is to eliminate those free swiming baterial and body...
3. third day - 100% water change and add metro @ 1.5gm per 100L of water + 100gm of salt...as you already got metro...it is better to use tetracyclin antibiotic...
4. fourth day - 100% water change again and use method same as No.2...this time is to eliminate the remaining egg of free swiming baterial which is meto is too week to clean the job..
5. day five - same as method No.3...
6. day six - same as method No.3...
7. final day - back to method No. 2 to cancel treatment...
please when use PP...please allow 2 to 3 hours before adding PP if yo're using anti-clorine...anti-clorine will fertilize PP and turn into pure or choco color......