Goldfish actually don’t become pregnant like most people always think they do. The fact is, just like any egg-laying fish species, goldfish actually breed and reproduce through external fertilization. What all these mean is that there are female goldfish that carry the eggs and the male goldfish with the milt/sperm and when they court each other to breed, the males will chase after the female goldfish getting her to release the eggs into the water. At the same time, the male will release the milt and get the eggs fertilized. Thus, this is what we call the term external fertilization because it does not involve injecting the sperm from the male fish to the female fish causing it to be pregnant with babies.
Going back to the topic on whether a goldfish can be pregnant, the answer is definitely no. In fact, other important information that you will need to know is that goldfish are lousy parents which means that they don’t look after their eggs. Once the female fish release her eggs into the water, both the male and female goldfish will greedily eat up each and every one of their own egg that they could find and thus experienced fish breeders will know that using plants is the best way to hid and conceal the eggs from getting eaten by their parents. You might want to read up further about fish breeding subject to learn more on how different species of aquarium fish actually produce their young.
So finally, how do you know that your pet goldfish is ready to lay eggs? There are a few indicators which you can use to determine this condition. One simple observation is by looking at the anal opening. A female goldfish full of eggs will have the anal area slightly bulging and sometimes you can even see tiny round material just outside of it. Female goldfish ready to breed will even show a full round appearance or it can even be off-shaped which are seen bigger than normal condition. Compared to the males which slightly thinner (but not really thin) this will help you to differentiate between the opposite genders. There are other methods like presence of tubercles appearing on the male which are more accurate. However, do not be deceived based on size if your goldfish happen to experience some of these conditions that would easily have the owner mistaken that their goldfish is actually pregnant.
Dropsy/Pine Cone Disease (Sometimes also known as Bloating)
This is a common goldfish disease that will cause the fish to bloat and the scales standing out like a pine cone. It is a very scary condition feared by hobbyist because it can be very difficult to treat and cure. More information about dropsy and how to deal with it.
Overweight and Overfeeding
Some of your goldfish might look like it is pregnant but in actual fact the expanded body size could be reason caused by overfeeding. There are others which are too spoiled and fed excessively making the goldfish look fat and with the rounded belly. Always avoid giving excess and too much food more than required.
Natural Origins like Ryukin & Pearlscale Goldfish
Certain goldfish type/variant like the Pearlscale can exhibit a ping pong ball-like figure which is very odd-looking and that might led you to think that it is actually having filled with eggs. But the fact is for pearlscales both males and females look more or less exactly the same with the rounded bellies. Same goes for all adult Ryukin with that strong and deep body coupled with the rounded belly. Therefore, it can be very difficult to tell if your ryukin is really going to have babies unless you observe closely.
To help you understand even better, I’ve even attached a few pictures and photographs together with this post of an egg-filled female goldfish so that you can roughly know whether you fish is ready to breed and produce young offspring. I just hope that this helps and wish you luck on happy goldfish collecting. More related information about caring for your baby goldfish and topic on goldfish reproduction.