Hi Cupid,
The 5 footer tank is more than enough to house your critters. I'm not sure about aqua one 2200 because I do not own one but i have been using Ehiem external cannister filters for many years without any problems, I've a 2215 which I've been using for almost 20 years without much wear and tear. If your cannister is lagging, it could be due to clogging or the head needs maintenance or changing.
I'm not sure about multi-cure but if its green colour it's usually malachite green. Try Tetra Medica, look for the range that cures external fungus and dose according to dosage regime, xxML=xxLitres. Your fish shall heal within the week if the dosage is correct. If you can't find Tetra, look for medication that heals external fungus and red lesions and continue with the salt regime.( according to your description, it is definitely not Ich or velvet but there are more than 1 infection here. )
PS:It is recommended to do a 50% water change before start of new medication regime. Pump more oxygen into tank, starve fish for 2 or 3 days. The critters should be jumping over each other when the next feeding comes.
Tetra Medica is good for 30 days before redose also look for medication that does not harm biological filter. ie:Waterlife
Good luck and god speed.
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