Goldfish overcrowding (also called “goldfish overpopulation”) is a serious cause of goldfish dying prematurely or having stunted growth.

I find it helpful to use the analogy of your own home. Imagine that you doubled the number of people living there. And then you couldn’t clean it for several days. How would it be? How would you react to that?

Oh yeah, you can’t open the windows or doors either. You have the same amount of oxygen as before. How would she feel?

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

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

Unfortunately, what they don’t realize is:

1. Pet stores sell out goldfish quickly, so they can keep large numbers, for short periods, in smaller aquariums.
2. Young goldfish (“fry”) are usually very small, so you can keep many of them together for a few weeks.
3. Goldfish aquariums are usually cleaned very frequently (daily) at pet stores to remove food debris and feces.
4. When you come home with the goldfish, they will start to grow very quickly. In fact, you may be surprised to wake up one day and see how much they have grown.

My rule of thumb: a 15 gallon tank for a maximum of 2 medium-sized goldfish. The bigger the aquarium, the better. You can never have too big an aquarium.

So why is overcrowding such a danger to your goldfish?

Let me give you seven reasons why:

1. Waste products from excretion reduce the amount of oxygen available and cause ammonia buildup.
2. This means that goldfish are poisoned by the water in which they live and “breath”.
3. As ammonia levels rise and goldfish continue to grow, there is less oxygen available in the aquarium.
4. Goldfish will either start swallowing at the surface or begin to develop a number of common goldfish disease problems.
5. They begin to suffer from a lack of oxygen.
6. Without enough room to move, breathe, or play, the fish begin to suffocate.
7. Eventually, the combination of stress and lack of oxygen leads to death.

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

It’s also worth noting that “goldfish stress” is a very real phenomenon and can be caused by any number of reasons. Again, even if you’re not agoraphobic, imagine you’re stuck in 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. It was so full that I backed myself against the wall: I couldn’t move. I remember stress levels went up rapidly…

Goldfish actually undergo chemical changes under stressful conditions. They are designed to release hormones that slow growth in harsh environments. And you don’t want to unintentionally create a “hostile” environment in your tank by having too many fish.

In the worst case, goldfish grow so large that they are physically unable to rotate in an aquarium. They get stuck in the same position. They cannot feed properly and become distressed.

Finally, your goldfish needs plenty of room to move, swim, and play. You can’t expect them to be happy if you don’t give them room to move. It will be much more pleasant for you to see them move gracefully. It gives them the opportunity to engage in a normal range of healthy behaviors. These are fascinating to watch (and watching goldfish has been shown to lower blood pressure as well).