How to Tell if Your Aquarium Fish Are Stressed?

While it may seem like they’re living a care-free life in the aquarium, being fed and cared for by their owners, aquarium fish can suffer from stress too.

And just like any living being, stress is bad for your aquarium fish as well.

Fish are prone to stress if they are living in less-than-ideal conditions such as a messy aquarium, inappropriate water conditions, or lack of good nutrition.

When that happens, your fish are at an increased risk of falling sick and succumbing to the disease. Therefore, it’s important to be able to recognize when your fish are getting stressed so that you can take timely measures to help them overcome it.

In this post, we’ll be discussing how to tell if your aquarium fish are stressed, how to deal with stress-struck fish and other things you should know about stressed-out fish.

How to Tell if Your Aquarium Fish Are Stressed

There are certainly visible signs of stress in fish that serve as early indicators and can guide you to take necessary actions.

Let’s take a look at all these signs and symptoms.

Erratic Swimming

When your fish are stressed, they will often exhibit odd swimming patterns.

If your fish is swimming frantically with particular aim or direction, rubbing themselves on plants, rocks, and gravel, crashing at the bottom of the tank or locking fins at the sides, they may be experiencing significant stress.

General Sluggishness

This is another very obvious sign of stress in the aquarium fish.

Your fish can start moving much slower than usual when they’re under stress.

Gasping for Air

If your fish start gasping at the surface of the water, it’s a clear sign that there is a lack of oxygen in the aquarium water and that’s what’s making the aquarium stressful for the fish.

Even though some fish will routinely go to the surface of the water, constantly resting at the surface is an indicator of low oxygen and high toxins in the water.

Fish swim to the surface of the water to grab as much oxygen as they can as the oxygen content at the top is the highest.

Cloudy Fish

Your fish can start appearing cloudy when they are stressed.

You may also notice that their eyes look glazed over. A common reason behind this is that you didn’t quarantine the first sick fish in time.

If just a single fish is affected, you should quarantine it immediately. However, if all the fish appear cloudy, you need to get to the root cause and fix it.


If your fish appear to be uninterested in food, they could be stressed.

Stressed fish show a lack of appetite even if they’re being fed their favorite meal.


While there are a lot of fish that like hiding under rocks, plants, and wood inside the tank if you have fish that normally don’t hide and are now doing so more often, you might need to be concerned.

Clamped Pectoral Fin

If your fish are under great stress, they might stop using their front side fins or pectoral fins.

Therefore, this is an obvious indicator that you need to find out what’s causing them stress and then remove it.

Other Unusual Behaviors

Other behaviors you need to watch out for include:

  • Staying at the surface
  • Rubbing up against aquarium plants, rocks, and other substrates
  • Lack of swimming or staying stuck around plants
  • Swimming upside down
  • Not moving when other fish are attacking them

What Causes Stress in Aquarium Fish?

Stress in fish can be caused due to a number of different factors.

Generally speaking, fish are quite resilient creatures and as long they are provided with fairly consistent aquarium conditions, they can tolerate some level of stress.

However, sometimes it can get a little too much for them. A change of habitat or a disturbance in routine and behavior can cause significant stress.

Here are some of the most common stressors that the fish face.


A lot of times, you can find it difficult to spot disease as most fish will not exhibit any symptoms until right before they die.

The best bet for aquarium owners is to keep the tank clean and to not introduce new fish without quarantining them.

Cycling the tank is also very important as it keeps the fish protected.


Unlike diseases like Ich that appears as white spots on the fish, a fungal infection affects larger areas.

It may start from the fins but can cover the entire skin of the fish.

Bacterial Infection

A bacterial infection may appear as a deep maroon or blood-red spots on the fins.

Other common fish ailments that you should keep an eye on include fin rot, gill mites, fish lice, and anchor worms, and dropsy.

New Tank Syndrome

Aquarium owners who skip cycling the tank and quarantining fish before beginning their aquarium journey end up putting tremendous amounts of stress on their fish.

Cycling of the tank allows beneficial bacteria to establish themselves and nitrogen levels to be stabilized.

If you don’t take care of this step, stress and disease will be unavoidable.

Poor Water Conditions

Before getting started with the tank, you should also make sure that the water conditions are optimal. For this, you need to test it for pH, ammonia, and nitrogen content (nitrates/nitrites).

Some level of fluctuation is normal and can be tolerated, however, a sudden and drastic change can lead to fatal consequences for your fish.

Regularly testing the water is necessary and if you observe any irregularities, you should be ready to do a partial water change.

It’s also important that water is changed partially because a larger change can further damage the fish. Make sure that the pH changes do not exceed above a sudden 2 pH spike or drop.

Most fish can tolerate smaller changes. Generally, you should try and ensure that the pH stays within the range of 6-8.2.

Another factor that affects different types of fish is the hardness or softness of the aquarium water. Certain fish types such as cichlids require special water conditioners.

Oxygen Level

Insufficient oxygen is usually a result of excessive biowaste creating contaminants being produced in the tank.

Poor oxygenation level of water will most definitely stress your fish out so if the level goes low you should quickly add an air pump and air stone into the tank.


A little amount of salt present in your freshwater aquarium is beneficial as it has sanitizing action. But too much salt can be bad and can easily kill your fish.

Therefore, you need to use it sparingly and closely monitor the salinity of the water.

You can do this by assessing the osmotic gradient of the water that represents the balance between the salt and water column.

Related read: Can Distilled Water Be Used in a Saltwater Aquarium?


Maintaining a constant water temperature is also very important for your fish’s health. Sudden swings in temperature can make them stressed.

Therefore, whenever you’re doing any water changes, make sure that the new water is at a similar temperature.

In addition, you should check the water temperature every day when you feed them. Keep an extra heater ready in case you need to replace the existing one.

Compatibility with Other Fish

Getting bullied by other tankmates can also be very stressful for fish.

Fish can not only be territorial, but they can also actually get pretty aggressive. And given the small space, things can get amplified really quickly.

Some fish start bullying all the other fish in the tank while trying to find their place inside.

And in that case, the only way to remove the stress is to remove the bully from the tank. Similarly, the fish that are not included in spawning pairs will also feel really stressed.

This also results in them being pinned up in one corner of the tank.

Dirty Filter

Check the aquarium filter to make sure it isn’t clogged with excessive dirt and debris.

This can cause the water to become stagnant and the extra biowaste produced releases deadly gasses that harm your fish. Thus, you should follow a strict filter cleaning routine.

Related read: How to Change an Aquarium Filter Cartridge without Losing Bacteria?

Some Tips on De-Stressing Your Fish

If your fish is suffering from stress, you need to take quick action in order to avoid any serious and possibly fatal diseases such as fin rot.

You should try and find out the cause of the stress and then remove it. You may also need to consult with your veterinarian if the condition of your fish doesn’t improve.

Cycling and quarantining are a must if you want your fish to stay healthy and stress-free. Equally important are regular water checks for detecting any changes in chemistry.

Also, make sure that the tank filter, air pump, and other equipment are working properly. Overstocking the tank must also be avoided no matter how much you love collecting various species of fish.

A denser population puts unnecessary stress on the fish and they can become aggressive.

To Sum Up

Even though many fish species are capable of tolerating minor changes in aquarium conditions, excessive stress can be tough on them.

Chronic stress can be deadly regardless of the fish species.

The sooner you notice the signs of stress, the better you will be able to take corrective actions.

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