Take the carbon out of the filter because it absorbs chemicals like fish medications. Throw the carbon away and don't put any more in until you have finished treating.
Why did you treat him for septicemia? That's blood poisoning.
Black patching as mentioned is usually cell damage similar to bruising on people.
The combination of problems with this fish could have an underlying cause. If the ulcers don't heal up and the fish continues to have problems I would be thinking it has fish TB (Mycobacteria). If the ulcers do clear up and the fish gets better then it won't be TB.
septicemia was about 2 and a half weeks ago,had lots of blood in his fins and was swollen all over ,first i treated him with myzxin,then interpet no9,today i have started the 2 nd round of no9 as advised, but was told to do a water change and run the carbon first( tho i am a bit behind,say a day due to the flu)
all the water tests have been fine apart from PH being down to 6.6 and GH being bit up( i assume to meds)((( just so you all know i wasnt guessing at meds or anything, i think i would have lost this fish ages ago if it wasnt for WILDER, she helped me from the min i noticed the first blood in his fins,and am very gratefull to her for all that she did)))
,as soon as he recovered he got black( tho only for a matter of hours back to normal now)
he`s not actually looking bad as he does in the pic,( shine is on him) hes def not gaunt,he is eating very well now and as active as when i first had him, the red patches are still there but not really doing anything do get worse or better.. this fish has been thru a hell of a lot,and i would hate to loose him now, all started when he jumped out of the bucket during a water change ..yes my fault
ps....too scared to try issolation, im not experienced enough to know what im doing, so i have been treating the whole tank...i have an empty 40 litre tank,spare heater,pump,filter( not cycled-but one that is in with my platys so i could with advice move things about)