Treatment: Aerate the tank vigorously to increase the amount of available oxygen in the water.  As a last resort for dying fish, the use of hydrogen peroxide may be warranted to bring about a rapid increase in oxygen levels.  This should only be done in emergencies.  Increasing the aeration is often sufficient.  In some cases, the gills of the puffer may be damaged, requiring increased oxygen levels during the healing period.  It is important to find the cause of the Hypoxia as quickly as possible and rectify it.

Background: Hypoxia is a condition where, quite simply, there is not enough oxygen reaching the puffer's tissues.  The cause of Hypoxia is not as simple.  The simplest cause is seen in fish during transport, as they exhaust the oxygen supply in the small volume of water in which they are traveling.  The other causes may be broken down into the following--

Insufficient oxygen in the water (environmental hypoxia):
* An increase in temperature without proportionate increase in aeration.
* Inadequate surface agitation/filtration for tank volume.
* Bacterial blooms due to buildup of detritus.  Neglected tanks can experience a surplus of "breakdown" bacteria, which consume more dissolved oxygen than the aeration can produce.
* Medications such as formalin and phenoxythanol will reduce the level of oxygen in the water.
* Metabolic demands such as fighting, spawning, and excessive flight (as per the fight or flight mechanism) can exhaust an already low oxygen supply.

Many diseases or illnesses, can create in the fish a physiological hypoxia:
* Infectious diseases.
* Nitrite damage to red blood cells.
* Gill damage, resulting from: acidosis/alkalosis, ammonia, chlorine/chloramine, chemical treatments, and/or mechanical damage from sharp objects. (as it is passed through the mouth and out through the gills)
* Pathogenic attack from protozoans, parasites, fungus, or gill flukes.
* Carbon dioxide poisoning.