Here's why:

* Fish respiration: Fish, like other animals, breathe oxygen and produce carbon dioxide (CO2) as a waste product. This process happens through their gills, where oxygen is absorbed from the water and CO2 is released.

* Metabolic processes: Fish, like all living organisms, undergo various metabolic processes that involve chemical reactions. These processes can produce different waste products like ammonia, urea, and uric acid, depending on the species and their environment.

* Hydrogen production: Hydrogen gas is not a common byproduct of normal biological processes in animals.

While some microorganisms can produce hydrogen as a byproduct of their metabolism, this is not the case for fish.