fw dips are dangerous. ONLY because you are highly likely to do it incorrectly. It is very difficult to maintian the proper ph in fw.
It's easier to do a formalin dip.
I take tank water, add formalin 3 AND methelyene blue with an airstone. I usually dip the fish from 30 mins to an hour at a time.
Remember, just because you can't see ich on the outside, doesn't mean that the fish is not infected.
If you can't do a med tank, then your best bet is just to feed, feed and feed. I soak the food in garlic and increase the feedings. If the fish are well fed and happy, they will fight off the infection naturally. Maintain your water quality via multiple water changes.