Male fish may have larger pectoral fins and small white spots on their heads and pectoral fins. Female fish may be larger and rounder than males.
There can be a wide range of differences between the genders of various species of fish. There can also be very few differences, making it difficult to discern which are male and female. Fish owners can look to a few common differences that may help them determine the gender of their aquatic pets.
Generally, females are larger than males and appear fatter. This is especially so when they are carrying a lot of eggs. In cichlids, the males also have a large head bump. Males of most species have a larger pectoral fin than the female, with a thicker main ray. Male fish at spawning time, usually spring, will develop white spots on their heads, gills and their pectoral fins.
If the fish are a species of spawning fish , it may be a good idea to wait until they start spawning to determine gender. This is when both males and females start developing more obvious external gender characteristics. Also, male group spawners will begin chasing the female around the tank as she starts releasing her eggs. This makes it easy to identify the fish in the lead as female and the others as male.