Article: With many fish, like koi, females tend to be larger than the males. This is especially true during spawning time, when females have large abdomens. To check the size of a fish in the same species, you can look at them when they're near each other, take a picture of each one, or catch one in a net to quickly use a ruler to measure its length and width. This is often caused by the fact that the female carries the eggs in her abdomen. Once the eggs are laid during spawning season, the females and males can return to the same size. If you catch a fish with a net to measure it, try to keep the fish in the water and place the ruler in the water to measure it. You will likely be able to get a general idea of the length and width. Look carefully at the face of the fish just between the eyes and above the mouth. If it has a large, protruding bump, there's a good chance that the fish is a male. This bump is called a "nuchal hump," and it's present on many types of fish, like the tilapia, angelfish, oscar, and discus. Some species of fish don't have a nuchal hump, but the presence of one is normally a great indicator that you have a male fish. Male fish tend to grow bristles, bumps, or shiny scales during breeding season to attract females. Females can sometimes also grow these attributes, but the male versions tend to be larger and more pronounced to attract attention. For example, male pleco fish will grow long bristles on their pectoral fins and around the edge of their mouth and head. While females can grow similar bristles, theirs tend to be smaller.

What is a summary?
Compare the size of the fish during spawning season. Look for a bump on the forehead of the fish to identify a male. Watch for designs and growths during mating season.