The limiting factor for fish undergoing hypoxia is the availability of fermentable substrate for anaerobic metabolism; once substrate runs out, ATP production ceases.  Endogenous glycogen is present in tissue as a long term energy storage molecule.  It can be converted into glucose and subsequently used as the starting material in glycolysis.
A key adaptation to long-term survival during hypoxia is the ability of an organism to store large amounts of glycogen.  Many hypoxia-tolerant species, such as carp, goldfish, killifish, and oscar contain the largest glycogen content (300-2000 μmol glocosyl units/g) in their tissue compared to hypoxia-sensitive fish, such as rainbow trout, which contain only 100 μmol glocosyl units/g.  The more glycogen stored in a tissue indicates the capacity for that tissue to undergo glycolysis and produce ATP.