Symptoms: Ick is visible to the naked eye in its encapsulated stage, looking like grains of salt stuck to the puffer's skin and/or fins.  While it is sometimes termed whitespot, it is has a raised appearance rather than a simple spot-like mark.  Sometimes this parasite is only present in the gills.  Puffers may exhibit labored breathing at the surface, or at the flow from your filter.  Other physical symptoms include darting and scratching against tank decor or plants.

Treatment: It is advisable that you read the "background" section below to fully understand the life cycle of this parasite.  This will aid in your treatment.  Read ALL the steps before beginning.

1. Quarantine the tank.  Be aware that in its free-swimming (tomite) sand stages  Ick is extremely contagious.  Also check the water quality.  Puffers in poor conditions tend to contract Ick more readily.
2. Add aquarium salt to aid in the osmoregulation of your puffer.  Salt also has an adverse affect on Ich.  Take note of your species' salt tolerance.  Use three (3) teaspoons per gallon(3.78 liters) as a guideline.  If you have a brackish species, you may use more (use discretion! you don't want to raise the salinity of your water past the tolerance level--check the parameters for your species).
3. Treat your tank with Maracide.  If this is not available, you can use malachite green, aniline green, and formalin.  Some other brand names are "Quick Cure" and "RidIch".  Please note that all of these chemicals are somewhat dangerous to puffers (it burns their skin), and the dosage should be halved.  Be sure to follow the directions for your meds, especially the performance of water changes (25-30% water change after each day of meds).  Treatments will normally be done every 3-4 days, but regardless of the directions, keep up the regiment for 12-14 days to completely eradicate this parasite.  Steps 2, 3 & 4 done correctly will work together to eradicate the parasite from your tank.  Discoloration of your water will occur, but is easily removed with fresh carbon when the treatment is finished.  *if you don't want to put the above chemicals into your tank, you may omit this section, allowing the increased salinity and temperature to take effect.  This, however, isn't the quickest way to treat Ick.
4. Raise the temperature of your tank to the maximum allowable level, taking note of the tolerance level of your species of puffer.  This speeds up the life cycle of Ick, allowing it to be killed faster as it brings the parasite into its tomite stage sooner.
5. Vacuum the gravel thoroughly.  While performing water changes, you may want to incorporate this step.  It removes some of the tomites from your tank that may have survived by falling into the substrate.
6. Watch your puffer for signs of secondary bacterial infection or fungus.  Treat with anti-biotics as needed.

*My preferred medication (along with the above steps) for Ick is Maracide AND either Maracyn or Maracyn-two.

Background:  Pufferfish are especially susceptible to Ick.  Understanding the life cycle of this parasite is helpful during the treatment, since it is only vulnerable during its free-swimming stage (tomite).  The parasite's life cycle is direct and simple.  The trophozites are found in pustules on the fish.  This is what we, as aquarists, see as the white spots on our puffers.  These trophozites mature, and escape when the pustules rupture.  It is at this point that the puffer is susceptible to secondary infections (such as bacteria and fungus), as the pustules leave an open wound.  The trophozites settle to the substrate, form a "cyst," and then reproduce asexually.  The cyst, now containing hundreds of "tomites" (the free swimming and vulnerable stage of Ick) breaks open, and the tomites search for a new host.  The tomite cannot live longer than 48-55 hours without a new host.  Upon finding a new host, the tomite burrows into the host's tissue and grows into a new pustule.  At the burrowing stage, Ick is referred to as a "theront".  The cycle then repeats itself.  This entire life cycle speeds up with increased temperature.  Since Ick is only treatable during its tomite stage, speeding up the life cycle means being able to kill them off sooner.  Treating ick depends upon the aquarist's ability to speed up the life cycle and kill the tomites.  Adding salt to the tank also helps, as freshwater Ick is a poor osmoregulator.  It is important to repeat the treatment every 3-4 days, perhaps more frequently if you are able to increase the temperature (every other day at 82F, 27.7C).  This will depend upon the temperature of your water, as you are seeking to kill the tomites swimming in your tank.  You will need to keep up the treatment for 12-14 days to ensure the eradication of Ichthyophthirius multifiliis. Vacuuming the gravel removes the remaining encysted trophozoites.

If you want to read more, here is some additional information on Ick:
    Ichthyophthirius multifiliis means "fish louse with many children"; it was so named because one parasite may produce as many as 600 to 1,200 offspring. The parasite penetrates the mucus coat and the upper layer of the epidermis. By these movements the epidermis is irritated and reacts by augmentation of the epithelial cells, resulting in a covering of the parasite by a layer of the skin of the fish. Thus the swelling is a pathological production of the fish skin as a reaction to the activity of the parasite and not the parasite itself. The Ich is always situated between the epidermis and the cutis, where it feeds on red blood corpuscles and disintegrated epithelial cells. The mature parasite, upon leaving its host, can have a diameter of 0.2 to 0.5 mm, although some larger specimens have been found of 1 mm. It sinks to the bottom of the water, where it secretes a soft jelly-like cyst. Here a series of rapid asexual divisions take place. The speed of this process depends upon the temperature of the water, warmer water facilitating faster division. While puffers are constantly being infected and left of parasites, the infection continues by the ever increasing numbers of parasites. The time the parasite stays on the fish is also dependent upon the water temperature. By raising the water temperature you will shorten the time it take the parasite to leave the host, but speeding up the time it take the new cysts to develop. This will be useful for medications that kills the cyst stage of the parasite. The young parasites cannot live long free swimming either. If they cannot find a host, they die with in a few days. According to experts, at 68 Fahrenheit no free swimming parasite can live more than 55 hours. If you were able to remove all the fish from a infected tank, the tank will be safe again after 3 day, But this is not a viable option. Complete treatment of a diseased fish will take longer, and you must treat the whole aquarium as there might be some parasites still present.  The cyst of Ich can often be found on water plants, which explains how these parasites can be smuggled into the aquarium by plants from a infected tank. (Axelrod, Shultz 1990)
    While some fish develop a natural immunity to Ick, this is rare in pufferfish.  Serum and mucus antibodies from immune fish immobilize free-swimming theronts in vitro , suggesting several potential antibody-mediated mechanisms of protection. For instance, antibodies in mucus could block penetration of theronts into the epithelium of the skin and gills. Because immobilization can be readily observed in the laboratory and fits a number of different models of potential mechanisms of immunity, considerable effort in this laboratory has been dedicated to identifying the target antigens responsible for this phenomenon, with the ultimate goal of developing a subunit vaccine.