Yup, that's right.....as long as you understand why and for how long.

Seems like parasites can overwhelm a fish over night. You wake up in the morning and the fish is grotesquely covered with little white spots or a slimy film. Adding medication is going to take a bit of time before it becomes effective, or the fish is so parasitized that he is just not going to eat your medicated food. You need to do something to decrease this situation immediately !

In a marine aquarium environment all the organisms have adapted to the salt density level that they reside in. Salt retains moister, and a specific level of salt retains a specific amount of moister. If an organism that retains a certain amount of moister is placed in an environment where the salt density in that environment is different then it is going to have to alter the amount of moister that it retains. The difference between these two moister environments is called 'osmonic imbalance'. The greater the difference in salt density the more radical and quicker that change occurs.

The physical structure of any organism is made up of layers of cells, or cell membranes. These cells and membranes have the ability to regulate the amount of moister they retain. If the difference in salt density / moister between the inner and outer layers of these cells / membranes is greater than can be regulated the result is a rupture within the cell / membrane tissue. This rupture will most likely cause a catastrofic failure and ultimately lead to the decline of the organism.

It is this radical change in salt / moister densities, and the inability for the cells / membranes to regulate that difference that we are counting on. The result is the smaller organisms are affected before the larger organism, or in other words, the parasites are effected first then the fish ( or its various organs and membranes ). What happens is the parasites ( the smaller organisms ) take on so much moister, that not only, their cell tissues and membranes rupture from the heavy transfer of fluids, but they simply explode due to the difference.

One thing to keep in mind is that we are trying to alter the balance between salinity's. The rest of the water parameters must remain the same....this means temperature and pH of the freshwater need to be the same as the saltwater the fish are coming from. This will require you to measure both temperature and pH, before hand, of the saltwater and duplicate that with your freshwater.

The really tricky questions is how long do you leave the fish in the freshwater....and that's a really good question. Three to five minutes is a good time frame to work with, and it all depends on how the fish reacts. If one looks closely you can sometimes actually see the parasite departing the body of the fish.

At first expect your fish to become motionless and begin breathing quite heavy. With a little bit of 'nudging' the fish will begin to swim around. Keep an eye on the fish as at some point he will stop swimming around. Once the fish seems to be stressing more than he was with the parasites it is time to remove him from the freshwater. Yes this is a bit stressful to the fish, but it is far better than the alternative. 

After this bath is over return the fish to his original environment. Now that the parasites have been decreased the fish will have a better chance. The next step is to begin a medicinal / chemical means of dealing with the balance of parasites, and the subsequent bacterial infections caused by the parasites.