Treatment: Partial water change as needed, giving the puffer a day or so to show improvement before executing additional changes.  Find the source of the high nitrate production and remedy it.  The addition of live plants is helpful in controlling nitrate levels.

Background: This type of poisoning has many causes.  Some of the more commonplace are: overfeeding, overcrowding, and irregular water changes.  Some local water supplies may have a high nitrate level to begin with, excacerbating this condition.  Plant fertilizers may also raise the nitrate levels to toxicity.
Nitrate Shock: This can occur when a new fish is placed in a tank with extremely high nitrate levels (greater than 25 mg/l).  The fish is often terminal before signs of this shock are exhibited.  The fish is often found dead, with "mysterious" causes.