What is Acute Hypoxia?

A sudden depletion in available oxygen at the tissue level

When a fish is removed from the water for an extended period of time, the gill filaments adhere to on another, and the gill lamellae collapse.  This causes an oxygen deficit and acidic carbon dioxide accumulates.  The pH level in the blood decreases and the heart rate slows, until the fish essentially passes out.

When returned to the water the heart action becomes tachycardic (beyond normal range).

The longer a fish is removed from the water, the longer it takes to revive them.