I agree you're dealing with uronema and possibly brook, and perhaps even other ailments. That looks like uronema to me as well. Unfortunately, it doesn't need a host to survive in the main tank, it's one thing that is a definite why everything should be quarantined.
I would argue that healthy fish remain unaffected by it. So if you quarantine all fish and get them fat and happy in the quarantine tank and then watch them for two weeks before addition you SHOULD be OK. However, there is a risk that uronema takes them out afterward. The other alternative is to tear it down and sterilize everything in bleach and start all over. Lots of biospira would be your friend in that case. Treat all fish, and treat all incoming fish and run "rock and anything wet" 76 days fallow before it enters your tank.
The parasite environment these days is so poor, this is a sad state of affairs and a tough choice to make.
