If You Recognize These Signs, Your Fish Might Be Stressed

Alison Page

Alison Page


Signs of Stress in Fish

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An aquarium should be a serene haven for you and the aquatic creatures that live there. You should be able to relax after a long, hectic day and enjoy your setup’s aesthetic tranquil beauty, secure in the knowledge that your pets are as happy and relaxed as you are.

However, aquarium fish are highly susceptible to stress, which can manifest in various ways. Understanding the subtle and glaring indications of stress in aquarium fish is crucial for ensuring their well-being and creating a harmonious underwater environment that both you and your fishy friends can enjoy.

Stress impacts the fish’s immune system, leaving it susceptible to attacks by parasites and bacteria. So, it’s critical for your livestock’s health that you provide a stress-free environment for them.

Keep reading to learn how to spot the telltale signs of stress in your fish and put things right for them!

Key Takeaways

  • Identifying stress signs in fish is essential; look for color fading, reduced appetite, erratic swimming, or hiding as indicators of an unhappy aquarium inhabitant.
  • Stress in fish can result from environmental inadequacies like undersized tanks, poor water conditions, and overcrowding, as well as social issues like aggression and overzealous breeding behavior.
  • Prevention and remedy of fish stress include proper tank sizing, regular water maintenance, careful species selection, controlled breeding, and mindful tank placement away from noise and disturbances.

How Can I Tell if My Aquarium Fish Are Stressed?

So, how do you know if your fish are stressed?

Like all living creatures, fish get stressed in response to environmental changes, social dynamics, and health issues. This is especially true if the fish are wild-caught since they have a whole new, artificial environment to adapt to.

Although some stressors may be unavoidable, you must be able to spot the signs of distress in your aquatic pets so that you can take action to fix the problem.

Color Changes

One of the most obvious indicators of stress in aquarium fish is changes in their coloration. Fish that are typically bright and vibrant in color often fade and lose their metallic sparkle when stressed.

Lack of Appetite

If you’ve ever suffered from stress, you’ll know that one of the first signs is loss of appetite. You just don’t want to eat anything; the same goes for your stressed fish.

Of course, not eating can also be a symptom of a myriad of fish health problems, so you need to do some detective work to find out why your fish have stopped eating. That said, stress is a common reason for fish to go off their food, and fixing the cause can be all it takes to get your fish eating well again.

Behavioral Changes

Yellow tang fish in aquarium

In the wild environment, fish can relocate if the habitat becomes overcrowded, food is short, or an aggressor is creating problems. Of course, in the confined space of an aquarium, the fish have nowhere to go if they are unhappy or stressed.

Some fish species begin to display odd swimming behaviors, darting around the tank and even trying to leap out of the water to escape the stressful environment. Others exhibit a behavior commonly called glass surfing. Glass surfing is when the fish constantly swims up and down the sides of the viewing panes as if it desperately wants to escape.


Catfish in aquarium

Many fish species can be quite shy and spend much of their day hiding amid bushy plants, in caves, or behind decorations. That’s quite normal and ordinarily is nothing to be alarmed about. However, if you notice a fish you wouldn’t expect to hide away, spending much of its time keeping clear of its tankmates, that’s a good sign the fish is stressed.

When fish are stressed, they often hide. That could be to avoid an aggressive or overly dominant tankmate, or it could be to get away from an amorous male.

Hiding sometimes happens when a fish is sick, injured, is guarding a nest, or is about to spawn.

Bullying Behavior

You might be surprised to learn that living in an environment where bullying is common is just as stressful for the bully as for its victims.

Semi-aggressive fish, like some gourami varieties, are sometimes territorial. Others won’t tolerate too many males in their habitat, while some can’t stand fish of the same species. Whatever the reason, inappropriate tank mates usually lead to bullying.

Before buying fish for your aquarium, research the species carefully to learn which companions get along well with each other and which don’t.

Fin Rot

Fin rot is a common condition affecting aquarium fish. A bacterial infection causes the disease, although poor water conditions and a stressful environment often trigger it.

Frequent Disease Outbreaks

As mentioned earlier, stress compromises the fish’s immune system, making it highly vulnerable to disease. So, if your fish seem to be constantly sick or fail to thrive as they should, stress is a likely culprit.

What Causes Stress in Aquarium Fish?

So, what causes stress in aquarium fish, and what can you do to prevent it?

Tiny Tanks

Goldfish in fish bowl with bubbles

If you think about it, fish in their natural habitat have plenty of space and freedom to move around. They can relocate locally to find food, look for mates, and avoid aggressors and predators.

Territorial fish, like bettas, need enough room to mark out a patch to defend and spend most of their days patrolling it. Schooling fish, including many tetra species, need plenty of open water swimming space, and those shy and skittish species must have ample hiding places to retreat to where they can feel secure and safe.

If your aquarium is too small to accommodate the fish species you’ve chosen comfortably, they will quickly become stressed, and you’ll notice the behavioral changes we mentioned above. In the longer term, the fish will fail to thrive and probably be affected by disease.

The Solution

Before you add any fish to your collection or start stocking a new tank, it’s essential to check the fish’s expected adult size. Most of the specimens sold in pet stores are juveniles with plenty of growing still to do. So, that cute inch-long goldfish will grow to reach 6 inches or even more in its first year or two, rapidly outgrowing a small tank.

You must also understand the species’ habits and buy a tank of a suitable size. For example, schooling fish need a large, rectangular tank with plenty of space to move around, and solitary types do best in a spacious tank that offers sufficient hiding places and areas for them to be alone.

The bottom line: buy a tank that’s the right size for the fish you want to keep.

Poor Water Conditions

Aquarium filter output of a tropical freshwater aquarium

The water is kept fresh and clean in the wild environment through natural movement and rainfall. However, in the closed environment of an aquarium, you are responsible for maintaining clean, safe water for your fish and other livestock to live in.

If the water in your fish tank is polluted by fish waste, decomposing plant leaves, and other organic matter, the levels of ammonia, nitrite, and nitrates in the water will reach dangerous levels.

Although some fish species are hardier than others, all fish are susceptible to ammonia poisoning, which will very quickly kill them. In addition, living in a dirty tank with high nitrate levels is very stressful for the fish, leading to outbreaks of disease and a general failure to thrive.

The Solution

Every fish tank must be fitted with an efficient filtration system regardless of its size and occupants.

The filter draws the aquarium water through it and across the filter media. Here, particulate waste is removed, and beneficial bacteria process toxic ammonia into nitrites and less dangerous nitrates. The cleaned water is then recycled into the aquarium.

You must maintain the filter properly to keep it working efficiently. Rinse the media in dirty tank water every couple of weeks to get rid of sludge. You’ll also need to keep the pump’s impeller clean and clear of obstructions. Every month or so, change the filter media according to the manufacturer’s instructions. Once a week or so, change around 20% of the tank water to dilute nitrate levels.

For my peace of mind, I like to test my aquarium water every week with an aquarium water testing kit. The levels of ammonia and nitrites should be zero, while nitrates can be around 20 ppm. If those parameters are exceeded, you likely need to clean or replace the filter media and carry out a partial water change. High levels could also mean your filter system isn’t working properly and might need repair or replacement.

Bear in mind that the filter must be powerful enough to circulate the full volume of tank water through the unit and across the filter media between four and ten times every hour. In a fully-stocked tank or one that houses dirty fish, like goldfish or Oscars, you’ll need a larger filter unit to cope with the amount of waste they produce.


Colorful tropical fish swimming in aquarium

Overcrowding is highly stressful for your fish. As mentioned earlier, many species don’t take kindly to having other fish in their space, and most appreciate plenty of room to swim. In addition, the water quality in your tank will suffer since the fish waste will likely be too much for the filter to cope with.

The Solution

Again, before adding new fish to your tank or stocking a new tank, carefully research the species to find their adult size.

If you keep livebearers, like guppies, Mollies, or Platies, you could end up with too many fish because of their constant spawning. In that case, you could give some of your excess stock to a friend or sell some of your juvenile fish to your local pet store. Alternatively, you might prefer to buy a larger tank.


Symphysodon discus

Some fish species are quite aggressive and will not tolerate the presence of others in their tank. In addition, male fish often don’t appreciate too many other males of the same species in the immediate vicinity, especially if there are females to compete with.

Outbreaks of aggression and bullying are highly stressful for both the victim and the aggressor.

The Solution

Again, the solution to this problem is to carry out plenty of research before you buy any fish and bring them home. Unless you intend to keep a single species tank with one special exhibit, you must make sure that all the fish you choose are suitable for a peaceful, harmonious community setup.

Too Much Love!

Tetra serpae (Hyphessobrycon eques)

Although it’s great fun to keep livebearers, the males often hassle the females constantly in an attempt to trigger spawning. That endless pursuit and producing multiple broods of fry is incredibly stressful for the female fish and often shortens their lives.

The Solution

The best way to prevent over-amorous males from badgering females is to keep one male for every three females.

I also find keeping the tank well stocked with bushy live plants and adding plenty of hiding places and caves works well since the female fish have plenty of safe spots where they can avoid the male’s advances when they’ve had enough.


Catfish from genus corydoras in a aquarium

Many fish need to live in groups of conspecifics, especially natural shoaling or schooling species like tetras and Corydoras catfish. This is largely due to the principle of safety in numbers, which keeps the fish feeling safe in a wild environment they share with predators.

Isolating these needy species causes stress.

The Solution

The obvious solution is to always keep groups of at least six individuals of the same species. However, take care not to keep too many males, as that can create problems. Once more, research the species carefully to find out what works best in terms of numbers and the fish’s preferred habitat.

Incorrect Tank Placement

Beautiful aquarium on the  table in the room

Although some fish are naturally curious, many are easily startled by loud noises, sudden movement, and the like. Additionally, fish are susceptible to temperature shock, which can be caused by hotspots created by shafts of direct sunlight hitting the tank or fluctuating water temperatures.

All those things will stress your fish.

The Solution

Ideally, you should site your fish tank in a quiet, low-traffic area, well away from direct sunlight, heaters, fires, air conditioning units, and drafts. The aquarium should be close to power outlets where you can plug in your lighting and filter units and should sit on level ground.

Keep the tank away from your TV and sound system, as vibrations through the water are also a stressor for many fish species.

Kids and Pets

little girl looking at fish in nano aquarium

If you have small children and curious pets, try to keep them away from your aquarium. Tapping on the glass and continually trying to grab the fish’s attention will send stress levels rocketing.

The Solution

Be sure to supervise pets and family members to prevent them from pestering your fish. The easiest way to do that is to keep the door to the room containing your fish tank firmly closed when you’re not around to monitor things.

Final Thoughts

Fish are highly susceptible to stress, especially those that are wild-caught, and it’s up to you to provide a stress-free environment where your pets can thrive.

Many stressors can affect your fish, including poor water quality, inappropriate tank mates, overstocking, incorrect tank placement, and a tank that’s too small. Fish suffering from stress can exhibit unusual behavior, change color, and stop eating properly.

Provide your fish with an aquarium that closely replicates their natural environment, be vigilant for the signs of stress we’ve outlined in this guide, and take steps to fix the problems to protect your fish.

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