two sick fantails

Hi All-

I really need some help. I've had two fantail goldfish for about five years
now, and have never had to deal with any diseases until now, and I'm unsure
what to do.

Two weeks ago the smaller orange fantail was sitting in a corner of the
tank. His fins had a white edging to them, and looked like something had
been eating at them. He wouldn't eat, and looked miserable. My guess was fin
rot, but having never experienced this first hand I wasn't completely sure.
I took him out for fear the other guy would catch whatever he had, and
treated him with Multi Cure. The multi cure has: 0.4g/L Methylene Blue,
0.425g/L Malachite Green and 2g/L Acriflavine. This seemed to work well, and
I put him back with his friend when his fins looked better.

Then I noticed a few days later the other fish was attacking my orange
fantail, and he looked sick again. So again I take him out to save him from
the bully, and now he looks happy again.

But now, the other fish has gotten really sick, really quickly. His fins had
the same white edging, looked ragged, there was a strange film hanging off
his gills, and a white growth happening on one of his eyes. Now his gills
are looking red, and all he does is sit at the bottom of his tank and gasp.
I've also removed him from the main tank and treated with the same
multi-cure, but nothing is happening.

What can I do? I get contradictory advice from every pet shop person I speak
to, and I really want to help my poor little fish.

I've noticed in the past few weeks what appeared to be tiny air bubbles in
the tank. Could this have contributed to my fish illness?

Does this also mean my main tank is now contaminated, and should I disinfect
it and start again?