salt isn't a bad idea regardless. Costia is a possibility, though that typically increases mucus production and is evidenced by dullness/whiteness in patches. Having spoken to you, you have not added anything new, nor have any wild caught fish. I would advise to kick up that water change routine to dilute whatever is doing this until you can get a better grasp on what is going on. If I can, I will dig through my big book of fish creepy crawlies and see if I can come up with any ideas for you.