3-4 minutes is not useful.
a bath of at least 6-8 mins work. sometimes i even go up to 10mins if the fish is handling it pretty well.
this method is extremely useful against external parasites such as flukes, but provides only temporary relief to ich and velvet. for ich and velvet, treatment must still be carried out and FW dips are NOT a cure.
but for external body flukes, FW dip is good and the flukes drop off after a few minutes in the water.
be sure to observe the fish the entire duration in the FW dip. if it lie on it's side, prop it up gently and make sure it's swimming. if it's showing signs of severe distress and discomfort, remove it immediately.
i've done FW dips countless of times using plain tap water without dechlorinating and matching of PH. although it's advised, i think it's rather unecessary.
FW dips are extremely stressful to the fish and a little change in ph is not going to affect it much. however i still strongly reccomend using RO/DI water and dechlorinate it with matching ph if you can do it. if in emergency, using tap water should be fine.
this topic on tap water and not matching ph etc is a very grey area and has been discussed many times in Reef central. so i will not discuss here. just take advice with a pinch of salt.