Final answer:
Fish excrete ethanol as a by-product of anaerobic glycolysis, which allows them to survive in low-oxygen environments.
Step-by-step explanation:
When fish undergo anaerobic glycolysis, the metabolic by-product they excrete across the gills is ethanol. This process is advantageous because it allows the fish to survive in environments with low oxygen levels. Anaerobic respiration, or fermentation, enables organisms to continue glycolysis and produce a small amount of ATP without the need for oxygen. By excreting ethanol, fish are able to regenerate NAD* and sustain their glycolytic pathways even when oxygen is not available.