Treatment: Correct the pH level in your tank.  Using the appropriate pH buffer (sodium bicarbonate) to raise the pH is suggested.  Acids not containing phosphorous may be used to reduce the pH, but partial water changes to bring your tank to neutral are recommended.  Correct the levels slowly, using partial water changes.  The rate of change must not exceed 0.3 pH units per day.  Acute acidosis/alkalosis is commonly fatal, but a return to the normal level for the puffer is the only remedy.

Background: Acidosis/alkalosis is caused by a pH outside the tolerance of the fish.  The level will vary from species to species.  Such levels are relative to the natural habitat of the puffer.
Acidosis: Sudden drops in pH are usually due to a depleted buffering capacity.  Slow decreases are normally the result of acidic byproducts of the nitrogen cycle.  Carbon dioxide bubblers can also increase the pH by producing too much carbonic acid for the buffering capacity to handle.
Alkalosis: While most pufferfish tend to prefer alkaline conditions, extremely alkaline conditions are problematic.  This is usually seen at a pH level of 10-11.  Acute alkalosis is only seen where puffers are moved from one tank to another of drastically different pH level, or adding too much alkaline pH adjusters.  Chronic alkalosis can be caused by neglect of ignorance, as evaporated water is replaced by mineral-rich water.  Soluble minerals seeping into the water from tank decor may also contribute to alkalosis.
pH Shock: In extreme cases where fish are put into new, improperly matched water conditions, it may simply sink to the bottom, listless.  It may land sideways, or even upside-down.  More commonly, no signs are seen until 18-24 hours later, which is terminal.  This is when your new puff is found dead the next "lights on" in your tank.