Three posts in a row.
Okay, so it's definitely the Furan making him worse.
Here's what I did:
Night One: Two packets (20 gallons).
Night Two: Two packets (20 gallons).
Night Three: 25% waterchange. Added fresh carbon.
I realized after this it said I needed to dose him four times total before adding Carbon. Oops. He was looking alright, but some cuts got worse while some fully healed.
Two nights after when I noticed two new cuts on his head with fungus growing around them. I took the carbon out and I gave him one packet of Furan. The next morning most of the bad wounds were looking white/black (his normal colors) and not red. His tail is pretty bad because most of his skin is missing. I tried giving him one more packet because I figured it was the medication making him worse. I woke up the next morning and all of his wounds were bright red again and looked fresh.
I'm doing a 25%-50% water change now. I won't put Furan in until I can see what happens with the wounds. If they get better, I'll stop dosing him. If they get worse, I'll try the single packet dose instead of full dosages.
I didn't think he was going to make it a few nights ago, but he's one tough fish. He hasn't eaten in several days either.