General Principle
Smaller fish generally have higher metabolic rates, so they burn energy faster and need more oxygen. This leads to a higher breathing rate. Their larger surface-area-to-volume ratio also means they lose heat and oxygen more quickly.
Other Factors
Gill size and efficiency, activity levels, species differences, age, water temperature, and water quality all influence breathing rate. Warmer water holds less oxygen, forcing fish—especially smaller ones—to breathe faster. Poor water quality increases breathing rates across all sizes.
While smaller tropical fish typically breathe faster, the relationship is shaped by many interacting factors.