Treatment: Ideally, remove the puffer to a properly cycling tank without excessive ammonia while the primary tank is being corrected. If this is not possible, the addition of one (1) gram of aquarium salt per two (2) gallons (10 liters) of water volume will significantly reduce nitrite toxicity.  Be sure your species of puffer is tolerant.  Alternately, subsitute filter media from a well-cycled tank, temporarily.  Nitrite levels will normally drop within 24 hours, depending on the size of the beneficial bacterial colony in the media.  Find the source of the excess nitrite and correct the problem.  Also test and treat for ammonia.

Background: High levels of nitrite affect your puffer's respiration.  It transforms the hemoglobin (red pigment in blood cells which transport oxygen) into methaemoglobin, which is an insufficient oxygen carrier.  High nitrite levels are usually accompanied by the presence of excessive ammonia.