Is My Aquarium Overstocked? (9 Signs To Keep An Eye On)

This is a complete guide to understanding the signs that can lead to an overstocked aquarium.

is my aquarium overstockedKeeping too many fish in the same tank can lead to serious problems. Overstocking a tank is a common mistake that many people make.

If you are new to fish keeping or an advanced aquarium hobbyist, it can be difficult to know if your tank is too crowded and overstocked.

Once you know some important things to look out for, you will understand if your tank has too many fish and therefore overstocked.

So I have compiled a comprehensive guide that will show you 9 clear indicators to look for that proves your aquarium too cramped.

Is My Aquarium Overstocked? Here is what you need to look for:

  • Stress
  • Disease
  • Bullying
  • Poor water quality – Nitrate, Ammonia, cloudy water
  • Stunted growth
  • Fish gasping
  • Algae blooms
  • Odd behavior patterns
  • Excess fish waste not being filtered

Why should you not overstock your tank?

is my aquarium overstocked?

When you add more fish to your tank and it becomes overstocked, not only it will be more difficult to maintain, but the fish will begin to suffer and their quality of life will decline.  It’s also possible to see more fish deaths because of it.

There is a rule of thumb that you may read online or hear in local fish stores that say to have no more than 1 inch of fish per gallon water. This is a horrible rule and it really only applies to very small fish. I have never followed this rule and neither should you.

Think about it, if you have a 10-gallon tank, would you put a 10-inch fish in it? Of course not!

The rule could apply for small freshwater fish like guppies where you could have 10 small guppies or tetras that are 1″ big in a 10-gallon tank. Other than that, it’s not a good idea to follow any of these “you can keep so many inches of fish per gallon tank size” as it just doesn’t work that way.

It’s important to look for certain signs that help you judge if your tank is overstocked. A fish tank should provide plenty of swimming space with several hiding places for fish if they need it.

The best judge of determining if your tank is overstocked is the fish themselves. The 9 indicators that I mention in this article will be shown by the fish and they will tell you if the environment they are living in is not suitable. 

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is my aquarium overstocked?

Signs that your tank is overstocked

1. Fish will become stressed

Fish can become easily stressed if their environment becomes overcrowded with too many fish. Stress can lead to a number of problems including disease and even death. Under stress a fish’s immune system becomes weak and it will be difficult for them to fight off any diseases.

Signs of stress include:

  • Not eating
  • Loss of color
  • Gasping
  • Not interacting with other fish
  • Hiding
  • Bullying
  • Injury
  • Illness

2. Disease

There are a number of diseases that a fish can easily attract in an overstocked tank. Common diseases include white spot disease, ich, velvet, tail and fin rot, swim bladder disease. It’s important to diagnose the disease early so you can treat it accordingly.

You should also have a hospital or quarantine tank ready to treat the sick fish. Using a saltwater quarantine tank can help you reduce infectious diseases, and cut off their impact of other fish and livestock around them.

3. Bullying

With the lack of swimming space available in an overcrowded tank, it is inevitable that bullying will take place. Fish need lots of free-swimming space to feel comfortable and unthreatened by other fish. It is easy to recognize if a fish is being bullied. Constant chasing, hiding in corners, competing for food, fin nipping, floating on their side are all signs.

Reducing the amount of fish in your tank and removing any bullies will help the overall stress and health of your tank.

4. Poor water quality – Nitrate, Ammonia, cloudy water

Poor water quality is a big indicator that your tank might be overstocked. It’s important at all times to maintain proper water conditions for your fish to stay healthy. If you have too many fish in your tank, these water conditions will begin to become poor.

Cloudy water is a sign that the overall bioload in the tank is not being maintained by the filtration. Too many fish equals a heavy bioload which will result in cloudy water. So to keep up with a heavy bioload condition, you will need to increase the filtration and/or remove fish.

Even if your aquarium filter system is sufficient enough to take care of all of the ammonia and nitrite, the end of the nitrogen cycle – nitrate will still go up faster than normal. When nitrate levels are too high, the fish’s immune system will suffer. It will make them more susceptible to catch diseases.

As you check your water parameters on a regular basis, you will notice that your nitrate and ammonia levels increase with an overstocked tank. Fish produce ammonia, and it is toxic. Adding too many fish makes it worse. Both nitrate and ammonia are harmful to fish and will cause illness and even death in severe cases. If your nitrate and ammonia levels were to spike, it’s a clear sign that your filtration is also not keeping up and your tank is overstocked.

During our regular maintenance, we perform water changes. In the case of an overstocked fish tank, the nitrate level will rise faster than we can remove.  Partial water changes might not be enough and more frequent water changes will be needed which means more work for us, and additional stress to the fish.

5. Stunted growth

It is very common to see fish growth to be stunted when tanks are overcrowded or fish are placed in too small of a tank. A huge mistake that hobbyists make is placing bigger fish in a small tank. This will result in the fish not being able to grow at a natural rate causing stress, illness and eventually death.

If your fish is not growing at the normal rate, seems stressed, and is showing behavioral patterns that are not normal, then your tank could be overstocked.

6.  Fish Gasping – Less Oxygen

Fish breathe dissolved oxygen through filtering the water with their gills.  Since there is a fixed amount of water in a fish tank, the amount of dissolved oxygen is limited.  When there are too many fish in the same fish tank, there might not be enough oxygen in the water to sustain them.

Gasping fish are showing a clear sign that there is not enough oxygen in the water and therefore your tank is overstocked.

7. Algae blooms

Too many nutrients in the aquarium water will have an effect on algae and cause them to grow and possibly bloom out of control. Algae feed on nitrogen compounds such as nitrate and ammonia.  

Keeping less fish will result in less nitrogen concentration in the water, and it takes away the necessary nutrients from algae. Lightly stocked fish tanks have fewer algae problems. Overstocked fish tanks will always have algae problems which is not good for the livestock in the tank. 

8. Odd behavior patterns

As you keep certain fish over time, you will notice that they have behavior patterns that are specific to them. Some fish are more active than others, some stay in certain areas of the tank, some will be more aggressive when eating and have food preferences, and many other things.

If your fish tank becomes overstocked, you will notice that these normal behavior patterns become not so much normal anymore. Fish can start hiding a lot, stop eating, not as active as normal, etc..

If you notice any abnormal patterns in behavior with your fish, this can be an indication that you have too many fish in the tank and it is overstocked.

9. Excess fish waste not being filtered

How many times have you seen a fish tank and the substrate on the bottom of the tank is covered in fish waste and poop? It’s quite often to see. If you have too many fish in your tank creating excess waste that can not be filtered out, then your tank is overstocked.

This can lead to serious water quality issues and the associated fish stress and diseases that go along with it. You can expect cloudy water and poor water quality for your fish to live in when you have excess fish waste not being taken out of the tank.

Avoid overstocking your tank – Don’t buy too many fish!

I know it can be difficult to not buy too many fish for your tank. This hobby is addictive and there are so many awesome fish on the market for you to have. So many people in the hobby will start with a smaller tank and then over time keep upgrading tank size as their fish grow or you purchase more.

More tanks and more fish! I get it. The addiction is real and the temptation is hard to avoid.

But now that you know the many downsides of having overstocked a fish tank, it’s something we all need to avoid. What you need to understand is the fact that you will do more harm than good to both your fish and yourself if you overstock your fish tank.

If there is a specific fish you really want, make arrangements to relocate other fish to make room for the new one. Always keep your tank less crowded and don’t add new fish knowing it is the wrong thing to do.

Another situation when a fish tank becomes overstocked is when fish breed. In this case, you need to relocate the extra fish produced to a different fish tank. Set up a new tank or offer them to friends or the fish stores. You might be able to sell some and make a few bucks!

So how many fish is enough?


This is just something you need to judge and take everything mentioned in this article into consideration. Fish come in different size and shapes, and even different metabolism. Larger fish will require more swimming room.

Plan ahead of time and know how large your fish will grow over time. Generally the less fish, the better. Making smart decisions on proper tank stocking will save you a huge headache down the road.

Overstocked aquarium filtration

A very common question that people ask is if my aquarium is overstocked, can I just add more filtration? The answer is no. A filter will not solve the many problems of an overstocked tank. You can add very large filters or multiple filters, and you really won’t solve the bigger problem of an overstocked tank.

Additional filtration can help offset the heavy bioload that is present with an overstocked tank, but filters don’t remove waste. They collect waste and convert it into a less toxic form.

An aquarium is either in a fully cycled and balanced state or it is not. Additional filtration is an advantage but not a cure to issues created by a heavily stocked aquarium.

Which one of these 9 signs do you watch out for the most?

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