I think you definitely should treat everyone. When treating a fish, for anything including ich, velvet, internal parasites etc, you should always always wash your hands. Use a spoon for food (I used a itty bitty thing, that came with my powdered baby food) and wash that between feeding different tanks. Everything should be sterilized with vinegar/water, or diluted bleach/water mix then rinsed very well.
I think what you could do, is take everything out. plants, ornaments, rocks. Rocks you rinse with bleach and water, then just water (thoroughly) or chuck. Ornaments same thing. Boil them in water, then let cool in a bucket of tap water. Live plants, put in tap water, in a bucket as sterilization. This can be done for a week.
Leave everyone in empty tanks/ 1 gallon containers. Daily water changes, and the medication will help. Wash your hands between feeding fish. I've had to do this when 2 tanks had an outbreak of ich. Act like it is contagious, even if it isn't, then you'll take extra precautions.