Ditto to what FTMMWS said. It only will help some of the ich spores pop off and relieve the fish a little. But take RODI water get it to temp and close to PH as your tank is. I like to do mine in a bag and let the fish float in the tank like your acclimating it when ya got it. Personally, not a huge fan of it though. I used to do it, but no more for ich. IMO at this point unless your going to do a full on QT of the fish, alleviating stress and providing a good diet with vitamins is better than anything. And catching a fish and shoving it into freshwater isn't gonna help with stress levels, or rid the fish of Ich.
I know its a debated topic, so just choose what side you trust in. I used to do FW dips and never lost a fish, but doubt it really helped much as Ich is in the skin, under the slime layer, where a 30 minute dip won't reach. But it may help rid the gills, which may be good.
