I would use the 30 liter tank to serve as a hospital tank. Net all the fish, put them in QT, use warmth (80F or so), aquarium salt and frequent siphoning of the bottom of the tank (it should not have any gravel) to see if you can't get rid of this.
Catfish (BTW what kind do you have?) and neons are sensitive to many meds for ICH, which is why I advise this treatment. Some ich medications can allegedly be used even on cats and tetras if used at half strength
. In all cases check the warnings and instructions and follow them to the letter.
I suspect you are having trouble ridding the tank of Ich for two reasons:
1. It is difficult to accurately dose this volume of water
. Even if you put the proper amount of medication in the tank, materials like substrate, rocks and filter media can adsorb the active ingredient out of the water, and treatment is ineffective. This, by the way, is why activated carbon should be removed during treatment. . .it takes the medication out as fast as it goes in.
2. Once you have cured the problem, the addition of new fish makes the whole tank vunerable to re-infestation. So quarantine new acquisitions
for a couple weeks before they go in the 60gal to be sure they are not infected. Unless, of course, you enjoy doing this!
Main tank needs to be left fish-less for a couple weeks to be sure all the parasites die out for lack of a host. They generally go through an enture life cycle in about ten days, but warmer temps should accellerate this. Raising the temperature in that tank during the fallow period will further insure success. Gradually lower the temperature to normal range after successful treatment.