10-15-2012, 08:28 PM
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I would raise the temp even higher, around 85F/30C, for one week. Do a major water change first, half the tank, and raise the temp a bit with this, then adjust the heater for the rest. Going higher to 90F would be the cure, but the cory will find this difficult and so may the tetra, not knowing which species.
I can't guarantee the temp to 85F alone will handle this, but it might. If you can get to a fish store, a small bottle of CopperSafe would be my choice for a medication. It is gentle enough for delicate fish (corys, tetra) but in my experience effective. I would not use salt, ever; the cory and the tetra will find this very stressful and the tetra might well die from it if the salt is anywhere near strong enough to actually deal with the ich.
Ich is caused by stress, period. And preventing stress is the best way to prevent ich or to eradicate it [though this is not 100% possible]. The higher temp as someone mentioned speeds up the ich cycle, and can even kill them at 90F. Keeping the fish un-stressed is important so they can naturally fight it off.