How do you tell a boy goldfish and girl goldfish?

You can tell the difference between male and female goldfish during spawning season. This is because both sexes develop breeding stars. These are small, white tubercles that appear on the gills and pectoral fins. These will only be visible during spawning season, so if it's not spawning season the fish will appear the same so this may not be the most reliable way to tell them apart.

Male goldfish tend to be more streamlined than females, with a longer and narrower body. They also have longer fins, particularly the pectoral fins. Female goldfish are often rounder and have shorter, broader bodies. Their fins are also shorter.

Another difference between male and female goldfish is the size of the dorsal fin. The dorsal fin of a male goldfish is typically larger and more pointed than the dorsal fin of a female goldfish.

Finally, male goldfish are typically more aggressive than female goldfish. They may chase and nip at the females, particularly during spawning season.