Okay, so in the 2.5g you should do 50% every other day and 100% by week's end. It's your call which you want to treat in, but you'll have to move him less if he's in the 5g. Also if you do the water changes like I outlined in the other thread you won't shock him anymore... using thermometer under running tap to match tank water.. hour long acclimation, etc.
Cutting is also not accurate. If you overdose you can actually cause chemical burns to the skin, and if you underdose then you will just breed a resistant infection. The problem was the dosage of meds.. at this point anything you've used you can't use again.
The salt also had to be predissolved and not just added right? Again, not dissolved can caused burns that look black.. Also want to confirm that this was the salt you used? http://www.apifishcare.com/products/...x?productid=49
If he does need meds you'll have to get something you haven't tried, and I will help you dilute it properly this time, but not so sure he needs it just yet..