i have never used a salt bath for fish, so i would say just follow the instructions on the box????
But i do know that gold fish do tend to have a problem with fungus more than other fish i have had this problem before and it sounds like that might be what you have.
i would treat with an anti- fungal
as far as the red hair alge i have a horrible problem with this a few months back i ended up pulling out all plants scraping off what i could then dipping them in a slight bleach solution..... sounds extream i know but it worked and if you do it right your plants will be fine, when i get home later i will post up the article.
Now to keep it under control, i keep a closer eye on my PH keep it sround 6.6-6.8 and that seems to keep it at bay