How do you tell a goldfish from boy to girl?

It's actually very difficult to tell the sex of a goldfish just by looking at them! There are no external visual differences like with some other fish species.

Here's why it's tricky:

* No external genitalia: Goldfish don't have obvious external genitalia that you can use to tell them apart.

* Similar appearances: Male and female goldfish look almost identical.

* Hormone-driven changes: The only way to reliably determine sex is during the breeding season when males develop breeding tubercles, which are tiny white bumps around their head and gills. These tubercles help them stimulate the female during spawning.

So, you can't really tell a goldfish's sex unless they are actively breeding.