Freshwater dips are a very effective treatment for some parasites. They are certainly not a "newbie myth". The myth is that FW dips are a good treatment for ick. Ick (cryptocaryon) is a microscopic parasite that embeds too deeply in the skin to be significantly affected by FW dips. However it may initially appear as though the fw dips has removed ick because the white spots that you see are really just the fishes response to the microscopic parasite. A fw dip may wash off those whites dots of mucus but the parasite is usually still there, alive and well. Doing a FW dip on a fish with cryptocaryon and then placing it back into an infected tank can actually make the problem worse because now you have washed off the fishes protective slime coat thereby making it more vulnerable to further infection.

There are tons of parasites that show up with wild fish not just ick. Here's a link to a pic of a very common fluke and one that I find in my FW dip buckets on a very regular basis. He's a nasty critter with a pair of hooks on one end to attach to the fish with and a pair of sucker mouths on the other end. He's also usually not visible on the fish also although it may appear as white spots (ick) on larger scaled fish.
http://www.fishdisease.net/fd/diseas...php?img_id=584
Oh, and the females produce about 40 eggs per day every day. So keeping this critter out of your tank isn't a bad idea.

The original question was how long to do a FW dip. It takes 4-5 minutes to knock off flukes and parasitic copepods/isopods so you want to do it at least that long. I usually do them for 5 - 15 minutes, longer if I forget that I have a fish in my dippin bucket (don't do that ;) ).

I pretty much dip everything although I will usually skip them on very small fish (as in baby fish), and some of the parasite resistant fish such as gobies and mandarins. I also often skip them on lionfish also. They do fine but sometimes they'll jump right out at you (SCARY). Depends on how brave I'm feeling that day. Catching a lionfish in mid air is tricky.

Alot of people will wait until their fish are covered in ick, barely staying alive, and then do a fw dip out of desperation. That's a real good way to kill a fish. It's important to do them while the fish os still strong and can tolerate the dip well. And again, they are not a recommended treatment for cryptocaryon (ick).

The freshwater needs to be temperature and ph adjusted and fish seem to do alot better if you provide aeration/circulation via an airstone. For most hobbyist the ph of the bag water should not decrease much if at all on the short trip home but if for some reason the ph were very low then you would want to acclimate first. FWIW, most fish I recieve have been in then bag 24 hours with little to no decrease in the ph. Of course some suppliers are better at packing fish than others.

HTH

Steve - I didn't think anyone was actually listening .