Freshwater dips work but if you are going to put the fish back into the same tank its pointless. Small fish don't seem to handle freshwater dips very well either.
There is no easy cure for ich. You are better off customizing your treatment based on the species of fish you are dealing with. If you have fish you need to treat, post which ones and I'll give you a reco on each.
i have a blue hippo tang that started showing signs of it. it's progressed over the days but he's still very active and eating well. i also started soaking the food in garlic extreme. i have a 125g with ~100 lbs of live rock a 40g sump with refug 4 green chromis 1 fire fish 1 clown and the tang. none of the other fish are showing any sign of ich, just the tang. he's about 2.5-3inches
Make the pH of the water the same, by adding buffers, and the temperature of the water the same as the water it came from before adding the fish to the freshwater dip. You also want to keep a close eye to see how it reacts as some fish may not handle it well. Tangs from my experience are usually OK with the FW dips for a while though.
But I don't know how effective freshwater dips are on ich. From things i've read, the ich bursts from the salinity change, but I have not seen that happen from my trials. I've instead seen better results with hypo, although you need a dedicated QT system for that as its not reef safe