Male and Female Goldfish
It is easiest to tell the difference between male and female goldfish by their behavior during breeding.
Stephen M. Meyer
Q. I've had my goldfish for about a year now, and after two months they started breeding. I know which ones are the males and females because when they started breeding last year the males got small white spots on their gills.
But now that breeding season has started again, I have a confusing situation. Now one of my female goldfish has these white dots on her gills and she chases the other females! I was wondering if goldfish can change sex. Can she become a he?
Or, is she sick? Please help. I thought it might be white spot disease, but it's only on the gills.
A. Although some fish can change sex, goldfish are not among those. Sex is fixed prior to birth.
You are absolutely correct: at breeding time mature male goldfish will develop breeding tubercles that appear as white bumps on the gill covers (opercula) and the rays of their pectoral fins. It is a sure way of telling males from females.
It turns out that watching which fish does the chasing and which fish gets chased is not the most reliable indicator of a fish's sex under all circumstances. As the mating frenzy begins and pheromones (chemicals that one fish releases into the water to chemically communicate with other fish) stimulate chasing behavior, some males and females may chase other males, and some females may join males in chasing other females. This may happen in crowded aquariums and outdoor ponds with many immature and semi- mature fish.
But nature sorts things out because only female goldfish can lay eggs and only male goldfish can fertilize them. The fish that play the wrong role simply expend energy. And eventually these animals do fall into their appropriate role, as you noticed this year. Your immature male goldfish from last year, which was a target of the chase (and so you assumed was a female), is now a chaser.
Returning to your observation of breeding tubercles, as your message indicates, these "bumps" do appear similar to the signs of white spot disease caused by the parasite Ichthyophthirius multifilis. And less aware goldfish keepers frequently begin using fish drugs needlessly to treat the non-existing disease problem. Not only is this wasteful, but it can contribute to creating drug-resistant pathogens in your home environment that not only threaten these freshwater fish, but your family as well.
You did not mention whether you had a successful spawning last year. When your goldfish start chasing, you can put a spawning mat in the tank. This accomplishes two things. First, it provides a soft cushion for the female goldfish to "bounce" against. This may spare her some serious physical damage (smacking against aquarium ornaments or the glass sides can really mess up a goldfish).
Second a spawning mat will allow you to remove the fertilized eggs to a safe aquarium where they can hatch unmolested by hungry goldfish. You can buy these mats in a good aquarium store or use any clean, soft, fibrous material.