How to Tell if Your Aquarium Has Too Many Fish?

If you’re fond of keeping fish, you may have run into this problem more than once.

Keeping too many fish in your aquarium is a recipe for disaster.

It not only halts the development of the fish in your aquarium but has them compete for resources.

From the perspective of a pet owner and a pet lover, that’s never ideal. However, sometimes, you’re too inexperienced to tell whether you’ve goofed.

Here are the signs to watch out for when you’ve overcrowded an aquarium.

How to Tell if Your Aquarium Has Too Many Fish?

When you have overcrowded an aquarium, these effects begin to manifest.

While some of them may not be visible, they tend to manifest in physical forms sooner or later.

Others begin to manifest from the very beginning. Watching for these signs and effects will help you identify overcrowding.

Then you can decide to split up some of the fish in your aquariums or buy a bigger one.

Short Life Spans of your fishes

The life span of a fish in an overcrowded tank tends to be smaller.

A fish that is allowed to grow in a large environment without any restrictions will grow beautifully.

However, one that is forced to compete for resources and space will experience stunting and spinal deformities.

Atrophied muscles and other health problems will result.

This is most likely to occur to juvenile fish that are still growing.

If some young fish in your tank are constantly sticking to their corners or are becoming lethargic, there’s your alarm.

If some of your fish unexpectedly begin to die, then you definitely have an overcrowding problem.

If this happens, make sure to identify signs of lethargy in other fish.

Also, track the health of your fish regularly to identify if the sudden death of many fish is an anomaly.

At times, a few old fish can die at once. That is simply nature taking its course.

If you’re not too sure about the situation, you can check with a veterinarian as well.

If you’ve purchased a juvenile fish that will outgrow its tank soon, it’s a good idea to transfer it as early as possible.

When it develops health problems and has stunted growth, the damage is likely already done.

Competition for Resources and Space

Competition for resources is common in the animal kingdom. However, it becomes a problem in human-built habitats when there is overcrowding.

Aquarium overcrowding leads to fish fighting for food in most circumstances. It’s not just about food though.

In packed aquariums, fish can often butt heads and fight for space as well.

They simply see other fish that take their share of the food as enemies. This can even lead to mass infighting between a fish population.

Other times, fish are angry because they don’t have corners to themselves.

You may have seen fish hanging out in certain corners of aquariums at times.

This is natural.

Sometimes you would want to retreat to your room as well. However, overcrowding leaves little possibility of that.

This leads to tempers rising and fish competing for territory as well. The best way to prevent this is to find a bigger tank for your fish.

You can also split them up into two tanks if need be.

A great tip to remember is that not just space is necessary. The fish that you keep together need to be compatible.

The species needs to be like enough so that fights don’t ensue. If you can get the opinion of a veterinarian or a fish store owner, that’d be the best course of action.

Low Oxygen Levels

More fish means less oxygen for them to share. You can begin to see a lot of fish dying when this happens.

Using an air pump usually solves this problem.

Aggression Among Fishes

Aggression is a natural side effect of too many fish in a tank.

Fish need room to swim.

If they’re denied that, they will bash into each other and try to compete for space. This will encourage aggression and bouts of territorial rage.

Water Quality Degradation

One thing that keeps the balance in any freshwater ecosystem is the nitrogen cycle.

Nitrogen resides in the proteins that the fish eat and their feces.

It’s necessary that this cycle keeps on going for a healthy aquarium. When fish excrete, the waste contains ammonia.

The bacteria convert this ammonia to nitrite and that is converted to less toxic nitrate.

This is the cycle in a fully functioning aquarium. It keeps the water fresh, and the fish healthy.

However, overcrowding throws a wrench in this process.

Accelerated Ammonia Production

Cramping fish into small places usually causes chaos with water quality. The water quality degrades because too many fish produce too much ammonia.

This affects the pH of the water and the nitrogen cycle. Hence, the bacteria find it increasingly hard to convert the ammonia into nitrite and then to nitrate.

Instead, this results in more ammonia being produced, thus raising the pH of the water. This leads to fish deaths.

No matter what the size of your tank is, you should regularly test the pH levels of your aquarium.

Regular checks, as well as checks when new fish are introduced into the tank, will help. Also, remember to check with a vet or a fish shop owner for tips on keeping fish.

Algae Boom

When ammonia and nitrates accumulate in an aquarium, the algae swallow it up.

This leads to self-sustaining algae break-out.

This can result in less oxygen for the fish and eventual fish death. You can see the algae manifest in the tank quite visibly.

The sides of the tank are crowded with algae; hence the visibility into it falls. The best way to avoid this is to keep the tank clean.

It lowers maintenance work to install a filter and just set it on autopilot.

Illness and Stress

Stress and illness are bound to break out when you have an overcrowded aquarium.

Fish by nature share their living quarters, but cramping them into a tight space can heighten tension.

A fish in a small cup will be very uncomfortable compared to a 20-gallon tank.

However, if you have dozens of fish in a small water tank, it would quickly descend into chaos as well.

When fish aren’t able to relax in a corner, constantly compete for resources, and fight, it results in them dying.

This usually begins with lethargy and then slow, but inevitable, death.

You can’t cramp a naturally free creature that has gotten used to flowing rivers and seas into a box.

You will have to give it space to swim so that it feels free and is well fed.

It’s not an added bonus to a fish’s life; it’s one of the basic features to get to swim freely.

Stocking an Aquarium Properly

While we may all like to think that we’re experts at stocking fish aquariums, it’s not true.

You can’t just throw a few fish in a tank, install a filter and forget about it.

There are a few rules and concepts that you’ll have to learn if you want to keep them alive.

Here are a few key concepts to keep in mind when stocking aquariums.

The One Inch Per Gallon Rule

This is perhaps the most well-known rule for stocking an aquarium. Introducing one inch of fish per gallon works as a rough estimate.

This leaves a lot of room for error though. It’s just a rule of thumb that will help you calculate how many fish you can stuff into a single tank.

There are a few problems with the rule including no considerations for modern filtration, plants, lighting, etc.

It also neglects the kind of fish that you are keeping. For instance, bigger fish create more waste.

Hence a single 10-inch fish will be much more cumbersome than ten, 1-inch fish. You will find it easier to maintain the pH of a tank filled with the latter than the former.

The reverse can also be true. A full-bodied 5-inch fish will create much more waste than a slender, 7-inches.

Hence, the 1-inch/gallon rule really is for rough estimates.

Another margin for error to account for is the size of the tank. Of course, it won’t hold exactly as much water as its rating.

A ten-gallon tank will hold about 9.5 gallons taking into account the sand, the rocks, and the plants.

Then, of course, there’s the size of the fish that also takes up some volume.

The rule should be treated as a way to make rough estimates and nothing more. It’s a foundation to build on rather than the entire enchilada.

Surface Area

Larger water surface areas increase oxygen exchange for fish. This is also something that the One Inch per Gallon rule ignores.

Tall and thin tanks may have larger surface areas than short and wide ones, even with identical volumes.

The shape difference between the tanks also plays a part here. Then there are aquariums with different levels built into them.

The 1-inch/gallon rule is very inaccurate for them.

A much safer calculation would be 1-inch of fish per 12 square inches of surface area.

However, the surface area calculation also assumes a lot of things. It assumes that most of the fish you put into the tank is slim and slender.

It doesn’t take into account the rounded bodies of certain fish that are kept as pets.

The main advantage of the rule over the 1-inch/gallon one is that it takes into account different aquarium shapes.


Filtration also plays a huge role in the fish that your aquarium supports.

If you have a top-of-the-line filtration system, it runs four times the volume of your aquarium.

This occurs every hour. This means that a ten-gallon tank will have a filter rated at 40 gallons per hour.

If you take a slightly less capable filter, you may have a less capable fish tank in terms of sustainability.

If you’re in doubt about how much your fish need, then you can go for a higher filtration count.

There’s no danger of you over-filtering water after all.

Water Changes

Changing the water the old-fashioned way is also a valid choice.

Just make sure that adding small amounts of freshwater and vacuuming up the fish waste is a better strategy.

Don’t introduce your fish into freshwater immediately after transferring them from dirty water. This will come as a shock to their systems.

If you keep these tips in mind, you’ll save your aquarium from being overcrowded.

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