Goldfish overcrowding (also called “goldfish swarming”) is a serious cause of premature death or stunted growth of goldfish.

I find it helpful to use your home analogy. Imagine you double the number of people who live there. And then you can’t clean it for several days. What does it look like? How would you react to that?

Oh yeah, you can’t open windows or doors either. You’re stuck with the same amount of oxygen in there as before. How would you feel?

Now imagine your goldfish. They are in an artificially constrained man-made space (aquarium). They swim in a limited amount of water and, like babies, eat in and out. But unlike babies, they cannot cry and their voice cannot be heard if they are in distress.

Many people assume that when they go to a pet store and see fifty baby goldfish in an aquarium, that means it’s okay to buy a whole bunch of goldfish. Then keep it in a small tank at home.

Unfortunately, what they don’t realize is:

1. Pet stores retail goldfish quickly, so you can keep large numbers — for short periods — in small aquariums
2. Young goldfish (“fry”) are usually very small, so you can keep a large number of them together for a few weeks
3. Goldfish tanks are usually cleaned more frequently (daily) at pet stores to remove uneaten food and feces.
4. When the goldfish comes home, it will start to grow very quickly. In fact, you would be amazed to wake up one day and see how much it has grown

My rule of thumb: a 54-liter tank for two medium-sized goldfish, max. The larger the aquarium, the better. You can never have an aquarium that is too big.

So why is overcrowding a danger to your goldfish?

Let me give you seven reasons why:

1. Excreting waste reduces the amount of available oxygen and causes a buildup of ammonia
2. This means that goldfish are poisoned by the waters in which they live and “breathe”.
3. As ammonia levels rise and goldfish continue to grow, there is less oxygen available in the tank
4. The goldfish will begin swallowing at the surface or begin to develop a host of common goldfish disease problems
5. They began to suffer from oxygen starvation
6. Without enough room to move, the fish begins to choke, breathe, or play
7. Ultimately, the combination of stress and oxygen starvation leads to death

Here’s a common mistake goldfish keepers make: Just because tank water looks clean or healthy for your goldfish doesn’t mean it’s clean. When a fish suffers from ammonia or nitrate poisoning, what happens is that the concentration of ammonia (a clear, colorless liquid) becomes higher. The water looks clear, but it is actually toxic. You can only be sure it’s safe by using an ammonia test kit — and making regular water changes.

It should also be noted that “goldfish fatigue” is a very real phenomenon and can be caused by several causes. Again, even if you’re not agoraphobic, imagine yourself crammed into a small space with a group of people. How would you feel? I remember being stuck on a train once on a hot summer day. The place was so crowded that I was trapped against the wall: I couldn’t move. I remember stress levels rising quickly…

In fact, goldfish undergo chemical changes under stressful conditions. They are designed to release growth-stopping hormones in hostile environments. And you don’t want to inadvertently create a “hostile” environment in your aquarium – by having too many fish.

In the worst case scenario, the goldfish becomes so large that it cannot physically turn around in the fish tank. They get stuck in the same position. They can’t feed them properly and they get sad.

Finally, your goldfish needs plenty of space to move around, swim, and play. You can’t expect them to be happy if you don’t give them room to move. It will be more interesting to watch them move gracefully. It gives them the opportunity to engage in a normal range of healthy behaviours. These are great things to note (and watching goldfish has been shown to lower blood pressure, too).