Two points usually not brought up in these posts...
Positive ID
Not every instance of white spots is ich.
If you elect to do an out-of-tank treatment, consider that it will be very stressful to the fish from the point to try to capture them until the moment they return to the tank to recuperate.
With this is mind, if you're going to go through the trouble and stress, confirm to the best of your ability what parasite/condition you're targeting.
mcarroll's Favorite Fish Links has lots of pertinent links to get you started, including a guide on microscopes that's probably not that great, but better than nothing. A $10 jeweler's loupe (or something similar) is supposed to be best for observation. Nothing fancy needed for parasites.
@Paul B has written a bit about this if you do some searching.
Health of the Fish
Increasing the health of the fish so they can successfully fight it off/obtain immunity is the best answer at this phase.
Make sure you can get that right –
healthy fish are your real goal here.
Better food is often the answer. Live food such as black worms and white worms, newly-hatched brine shrimp, etc are the best. Frozen whole foods would be next-best. Blended foods are good, but I'd want to have some probiotics in the mix. The rest should be considered snacks.
Don't add any new fish until these guys have been healthy
without issue for several months and you should be OK.