Goldfish Biology

24. How do I tell what sex my goldfish are?

It is difficult to tell the sex of goldfish when they are not in breeding condition. When they are in condition, it is the males that are immediately identifiable. They develop something called "breeding tubercles". These are little protuberances that develop on the leading ray of the pectoral fins and on the gill opercula (the operculum is the large scale covering the gill). The tubercles look like small grains of white sand. On fish with headgrowth the tubercles on the gill covers are often not visible. But the ones on the pectorals are usually pretty obvious. Of course they are hard to see on white fish, but if you get sort of above and forward of the fish, you can see them as a serrated edge on the front of the pectoral fins.

I have seen males develop these tubercles as early as 4-months of age, though I would say 8-months to a year is more typical.

What about females? Well, they don’t have the tubercles and males (with the tubercles) will follow them around and nudge them in the rear part of their bodies when they are in spawning condition. The males can be quite relentless and even kill a female, if there is no way the female can get away and rest.

I have actually gotten a couple of notes from people saying that they had a sick goldfish, but that the other goldfish in the tank was trying to help the sick fish by nudging it to the surface. Uh, sorry. The "other fish" is a male and the owner simply has not observed the hours of chasing that exhausted the female, reducing her to an inert, bruised mass.

For some additional, more creative ways to try to figure out the sex of your fish, see the additional information in my article on this subject.

25. How do I breed my goldfish?

Goldfish are temperate climate fish. They are evolutionarily equipped to go through a dormant period in the cold months. As a result, they tend to store fat in the fall to see them through the winter. In the spring, among other things, they come into spawning condition as the water temperature rises towards 20C (68F).

Based on this, the most dependable way to get your goldfish to spawn is to allow them to go through a period of at least a month or two of cool temperatures with minimal feeding. A temperature of 10C (50F) or lower is great.

A number of people, myself included, have noted that fish kept strictly indoors seem to want to spawn in the December-January period. I don’t know why this is, but keep your eyes open and you may get lucky. Spawning usually starts before dawn and lasts several hours. Females also usually will spawn multiple times over several weeks or even months.

One critical element of this is spawning media; the females often will not release their eggs unless there is something suitable to catch them. It needs to be something that the fish can swim through. Real or plastic plants, mops of acrylic yarn, nylon thule, even bunches of shredded plastic grocery bags tied to a weight work well. I have found adding spawning material to a tank where the male is driving the female can often trigger the actual spawning within an hour sometimes.

For details about taking care of the eggs and raising the babies, please read the other info on this site or even better buy a book that deals with the subject in detail.

26. Are these eggs/babies in my tank?

Look at the pictures on this web site. The eggs are separate whitish spheres that stick to things, and are about a millimeter in diameter. Goldfish usually lay a lot of eggs (hundreds to thousands), but they also greedily eat them, so once the active spawning is done, they may eat much of the evidence. The babies are tiny little slivers of things that hang on the glass of the tank for the first few days before trying to swim much. The parents also eat these fry, if they missed them as eggs.

27. Why is my goldfish changing color? How do I stop it?

Goldfish can go through several color changes in their lives. I believe the color changes are genetically controlled, though I would guess environmental factors could have some effect. I just can’t tell you what effect. If the fish seems otherwise healthy, then it is probably a normal occurrence for that fish.

As an example, almost all goldfish start out as babies with the ancestral wild coloration. And then over a period of months or even years they change to their adult coloration. As they become older fish, they may change color yet again. I have heard from several people who have bought red Comets that have turned white in their old age. I have heard from many people who bought baby Comet goldfish that were orange with black edged fins, and who were very disappointed that they now had all orange fish. Well, they bought a fish that was transitioning from baby to adult coloring. Sometimes it only takes a few days.

There are a few color patterns that are almost always unstable. Basically, any goldfish that is a combination of black and either red or white will eventually turn, respectively, all red or all white. It may take a year or two, but it will happen.