Tazman, I think in many situations you're right with the advice you've given. HOWEVER, in this siutation I'm concerning about the Corydoras... they don't do well with salt OR high temps, much less both together. The high temp/salt bath might just kill them, whether they have Ich or not.
So... what to do? Since I'm generally against chemical treatments unless absolutely necessary, here is what I would recommend- remove the Corydoras to a bare bottom hospital tank*. Treat the main tank as Tazman suggested with salt (but use only 1 TEASPOON PER GALLON BUILT UP OVER THE FIRST 8 HOURS) and high temps (86 derees F) and aggressive gravel vac, but also add extra aeration
. The higher the temp of the water, the less its able to hold oxygen. We don't want to stress the fish out any more than we already are, so use an airstone or lower the water level so that the filter creates more splashing.
Then treat the hospital tank with a formalin/malachite green drug like Kordon's "Rid-Ich" or Aquatronics' "Formalite II" or Mardel's "Maracide" (see link below) exactly as directed
. Be sure that formalin and malachite green are the only ingredients. Put the tank hospital in darkness by covering it with a blanket. (Light lessens the efficacy of these drugs.) Be sure to siphon off the bottom of the tank daily.
Continue BOTH treatments for 3 days after the last cyst drops off.
(*If you don't have a hospital tank, let us know and we can help you rig one up cheaply and quickly.)
Know your enemy: Ich | The Skeptical Aquarist