I have found most species of corydoras to be very hardy. I would actually consider dwarf gouramis to be far more delicate. I absolutely agree that treating the fish seperately would be ideal. In fact, I wouldn't run an aquarium w/o a QT available, and salt ruining plants is definitely one of the reasons. But if you don't have a QT, and it's worth it to you to save the fish, I would pick up meds for it as soon as you can. I just wouldn't expect it to clear up on it's own, if at all. If no treatments are available to you all you can really do is frequent PWC's. Good luck, I hope it pulls through.