Overstock a fish tank with too many fish is a common mistake in fish keeping.  Many fish keepers have bought more fish than their fish tanks can handle.  Once a home aquarium is overstocked, not only it will become harder to maintain, but it will also make it uncomfortable for the fish and lower their quality of life.  In severe cases, you might also suffer higher fish loss because of it.

In the photo, the fish tank is overstocked with too many blood parrot cichlids.

We should always avoid putting too many fish in one fish tank for the following reasons

1.  Too many fish will use up all the dissolved oxygen in the water.

Fish breath 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.  While an air pump and airstone will increase the dissolved oxygen in the fish tank, there is still a limit to what it can do.

2.   Too many fish will produce too much ammonia

Fish produce ammonia, and it is toxic to themselves.  We can’t avoid it.  While the aquarium filter system takes care of ammonia through aquarium nitrogen cycle, every filter has its limit.  When there are too many fish, the filter might not be able to convert all of the ammonia to nitrate on time.  As a result, you might get ammonia and nitrite spike in the fish tank.  Both substances are toxic to the fish.  Even short-term exposure can cause permanent damage to the fish.  If the problem persists, some if not all fish will die.

While upgrade the aquarium filter might solve the ammonia problem, higher rated aquarium filter also comes with higher water flow.   If the filter is rated far above the fish tank size, then the water current might become too strong for the fish to be comfortable.

3.  Too much nitrate might lower fish immune system

Even if your aquarium filter system is sufficient enough to take care of all of the ammonia and nitrite, the final product of the nitrogen cycle – nitrate will still go up faster than usual.  When nitrate concentration is too high, the fish immune system will suffer.  It will make them easier to catch diseases.

Usually, we do the partial water change as a part of the weekly aquarium maintenance.  In the case of an overstocked fish tank, the nitrate will rise faster than we can remove.  Weekly partial water change might not be enough.  To keep the nitrate down, more frequent partial water change will be needed which means more work for the owner, and more stress to the fish.

4.  Algae boom from too many nutrients

Algae feed on nitrogen compounds such as nitrate and ammonia.  When the water is rich in nitrogen, it is hard to avoid an algae infestation.  Anyone who has kept a fish tank for a while must have seen algae.  It is hard to get rid of them.   They just keep coming back.

Keep less fish will result in less nitrogen concentration in the water, and it takes away the necessary nutrients from algae.  As a result, lightly stocked fish tanks have fewer algae problems.  It lowers the maintenance work and makes keeping fish easier, and more enjoyable.

5.  Fish might get stressed when there is not enough swimming room

When there are too many fish in one fish tank, it becomes crowdy.  Fish need swimming room.  Too little swimming room is no fun, but it will stress the fish as well.   If the fish are stressed all the time, it can affect their immune system and become sick easily.

6.  Fish might get aggressive if there are too many fish in one tank.

Some fish might get aggressive in a crowdy environment.  They will get frustrated with too many competitors for food and swimming room.  A few of them might start to nip at each other which will cause injury or even death.

7.  An overstocked fish tank will cost more to maintain

Not only you have to feed more fish, but you will also pay more for the maintenance supplies.

More fish means more mouths to feed.  You will go through the same bottle of fish food a lot quicker if you have to feed 30 fish instead of just 10.

In the case you have bought a higher rated filter for the heavy load, it is not only more expensive to buy, but it will also cost more electricity to run.

Aquarium maintenance at the very least will take aquarium water conditioner and water.  To keep a heavily overstocked fish tank at acceptable water quality, it will take more than just once a week partial water change.  As a result, it will take more water and more water conditioner to do the job.

8.  Too many fish is just not enjoyable.

Sure, some of us love to watch a fish tank with many fish.  I am one of these people.  However, the number of fish in a single fish tank that is still pleasing to the eye has a limit.  I do enjoy watching a school of Cardinal Tetra, or Neon Rainbow swimming together, chasing each other around, and display other natural social behaviors.   But when there are too many of them in a fish tank without enough space, all these natural behaviors will disappear.  There is just not enough room for them to do what they naturally do.

At this point, all you see is just a large crowd of fish.  They fill up every corner of the fish tank.  And every one of the fish is not doing much besides trying not to bump into each other.   What fun is there to watch something like that?

Learn to know the limit.   Do not get too many fish.

Now we have discussed many of the downsides of having overstocked a fish tank with too many fish.  It is something we all need to avoid.  I can understand it might not be so easy for some people to avoid the temptation to keep adding more fish to their already fully stocked fish tanks.  What these people should do is first to have a clear understanding that you will do more harm than good to both your fish and yourself if you overstock your fish tank.  Second, control yourself from getting more fish is essential.   If you can’t, try to avoid visiting the pet store, or the aquarium fish section of any store might be helpful.

There are also other situations when a fish tank become overstocked.  Such as when the fish give birth to more fish.  In this case, you need to relocate the extra fish produced to a different fish tank.   Add more fish tanks is a temporary solution.  We can’t just keep doing it.  Or before we know it, we might end up with more fish tanks than friends.  Give away the extra fish to relatives, friends, neighbors, or even the pet stores is a good solution.  Sell them if you want.  Some of the local pet stores will buy your fish.  It will at least give you some of the cost back.

If you do not wish to have the fish fry overpopulate your fish tank, you can do so by allowing the natural population control take place in your fish tank.  Some of the fish species will eat their fry.  In which case, just to make sure there is no hiding places for fry will do the trick.  If it is a species of fish that ignores their fry, you should have a few other species of fish in the same tank for population control.   Do not feel too bad about it.  It happens every second in the wild.   Most fish will swallow everything that can fit into their mouth.

Some people might ask, how many fish is enough.   It is a tricky question.   Because fish comes in different size and shapes, and even different metabolism.  In general, bigger fish will require more swimming room.  They eat more food, and they produce more ammonia.  You can fit a higher number of smaller fish into the same tank than bigger fish.   Do not use the current size of the fish as a measurement.  We should always research each fish species we plan to get.  Get to know their full adult size is essential.   Generally speaking, please use your common sense.  The less fish, the better.   For full details on the more precious standards, we will leave it for another article.