Tetras in red light

What Is Stress in Fish: Definition, Signs of Stress, Prevention

Stress is an intrinsic part of life. It is essential for growth and development, but it can also be detrimental and even fatal. That’s true for complex mammals like humans and for vertebrates like fish and other aquatic animals.

Stress in fish is the physiological reaction to threatening situations. In moderation, it helps fish grow and develop. Excessive stress — prolonged or intense, or both — can stunt growth, weaken the immune system, and impact behavior and reproduction. Ultimately, it can lead to premature death.

When building our wet ecosystems, the goal isn’t to create a completely stress-free environment but to establish a balance. Balance is not the lack of tension, but the dynamic equilibrium between opposing forces.

A well-balanced tank should provide enough stimulation for its inhabitants and also enough possibilities to rest and recuperate. Stimulation includes places to explore, proper company, some competition for food, and adequate water flow. Hiding places and enough room for all the fish constitute the places for rest.

Add to the mix quality water parameters and your fish will be happy campers.

Only that getting everything right is much easier said than done, especially when you are starting out. That’s why it is essential to understand what the main signs of stress in fish are, how stress impacts them, what you can do to prevent it, and how to treat stressed fish.

Stress Symptoms in Fish

  • Frantic swimming
  • Lethargy
  • No appetite
  • Whitening of color
  • Fin damage
  • Rapid gill movement
  • Gasping at the surface
  • Glass surfing
  • Excessive hiding

The signs of stress in aquarium fish are varied. Some are more pronounced than others, and some are more indicative of unhealthy levels of stress.

Most stress symptoms are not that radically different from those of a fish in shock. In both cases, we are talking about a physiological response to unfavorable conditions. The main difference is that shock impacts fish instantly, due to a sudden and drastic change in their living conditions, whereas the stress symptoms appear over time.

It is important to observe your aquatic pets from the moment you introduce them to your tank. Get to know them as much as you can. Observation breeds understanding and familiarity, and both will help you detect unusual behavior and signs of stress.

You see, fish are tough little critters. When the environment starts deteriorating, their bodies fight against the negative conditions. If the harmful influence is soon remedied, fish recover to their pre-stress condition. Otherwise, the stress starts wearing them down.

But let’s break down the signs of stress.

Frantic Swimming

All fish have particular swimming patterns and rhythms. Watch them for a while and you will see for yourself.

If they start darting around for a prolonged period of time, something’s probably off. I’m not talking about a couple of fishes playing around and chasing each other. Rather, a single fish dashing here, and there, and everywhere for some time, without any obvious purpose.

Glass surfing is a particularly noticeable abnormal swimming pattern. A fish would swim like crazy up and down the glass, relentlessly and seemingly forever. 

It’s probably trying to find a way out, to escape the thing that’s bothering it.

The frantic swimming symptom is hard to be detected in danios. They are the very definition of hyperactivity. For all their boundless energy, they tend to swim close to the surface, usually at the length of the tank. The pattern is there, despite the constant swift movement.


If a fish is considerably less active than the others of its kind in the tank, it is probably having a hard time. It may be lying on the bottom or barely swimming, possibly with dorsal fins drooping.

It may also be sluggish and disoriented, only to dash lightning-fast to another spot in the aquarium… Only to be limp and lethargic there.

Such unusual behavior is a strong sign of stress.

Lack of Appetite

Fish that don’t eat aren’t well. As simple as that.

Like most animals, fish are voracious eaters. They can overeat easily. If a fish stops eating food it generally enjoys, that’s a big warning sign.

To confirm that the fish is struggling to cope with stress, offer it another type of food that you know it likes. If it doesn’t eat again, then you should take measures.

Paler Colors

Many species react to stress by secreting extra mucus. To the naked eye, that looks like paler colors.

This stress symptom is more noticeable in darker fish because of the contrast their natural colors present. When things become particularly grave, the extra slime can become plainly visible.

Be mindful that there is something called slime disease as well.

Fin Damage

A clear indicator that something’s off with the fish’s surroundings is damage to the fins. Split or ragged fins impair the fish and make them look sick.

Fins can be damaged by excessive swimming (if the current is too strong or another fish is harassing the sufferer too much), high levels of ammonia, and ailments like fin rot, among other things.

Rapid Gill Movement

Any form of erratic breathing is a warning sign. It can be quickened, rapid gill movement that visibly betrays high levels of stress. It can be labored, slow inhales that make you want to breathe with the poor fish.

Or it can be…

Gasping for Air at the Surface

Some species, like betta fish and gouramis, like to have the occasional mouthful of dry air. Even most corydoras would venture occasionally to the surface of the tank to get a lungful.

But if a fish is sticking its mouth out of the water for an extended period of time, then probably something is bothering it.

Is it low oxygen?

Or is it too hot?

Excessive Hiding

Some species like to hide, so don’t be too quick to judge. But others don’t and if a single member of their school or shoal goes hiding, then something isn’t right.

It may be a symptom of stress or disease. Often, hiding when the rest of the species are foraging or swimming around can coincide with lethargy and lack of appetite.

Causes for Stress in Fish

Stress in fish is always environmental. As far as we can tell, they don’t have overly ambitious plans for career growth or for achieving inner piece. As long as their basic needs are met, fish will thrive.

But the basic needs are diverse and numerous and many things can go wrong.

Here are the main causes of stress in fish.

Poor Water Conditions

  • Dirty water
  • Dirty filter
  • Insufficient oxygen
  • Excessive ammonia
  • Elevated nitrate levels
  • Improper pH levels
  • Improper salinity level

All of the above can stress your fish out. The first two, dirty water and filter are extremely likely to cause all the rest.

The thing is that cleaning your filtration system has to be done carefully. If you have two filters, clean one at a time. If it is a single filter, always wash it in the aquarium water you are taking out. If it’s a sponge filter, rinse it but don’t go overboard. Washing it too zealously will remove too many beneficial bacteria, which can lead to ammonia spikes.

However, ammonia and nitrate levels may rise above healthy levels with seemingly excellent water conditions as well.

That’s why it is important to test your water chemistry. Ammonia is by far the most common killer of fish in tropical aquariums. It is produced mostly from fish poop and uneaten food. Dealing with ammonia and its impact on the water column is a large topic, but I have a few pointers on how to tackle elevated ammonia levels in the prevention section of the article.

Partial water change helps with most of the other stressors listed here. The main exception is insufficient oxygen. If your filter introduces oxygen into the water column, cleaning it will help with the airflow. In case it doesn’t, get an air stone. It is an affordable and aesthetically pleasing solution.

Live plants help mightily with good oxygen levels too.

Fluctuating Water Parameters

  • Unstable temperature
  • Inadequate light

It is important to maintain a stable temperature suitable for the fish you have. For tropical species, the usual range is 24-28 C, but there are exceptions.

As for the light, its intensity and duration are more impactful on the plants, but it also affects the fish. The main thing is to maintain stable periods of light and darkness. Turn the lights on and off at the same time every day.

A timer helps a bunch in that regard. It gives you freedom (late night out?) and eliminates the human-error factor (overslept on a Sunday?).

Inadequate Tank Size

Inadequate tank size refers to the sheer volume of the aquarium but also to the number of its inhabitants. Make sure to buy fish that won’t outgrow the tank in a few months. Not all fish are avid swimmers, but everyone needs a certain amount of space to feel comfortable.

If you end up having a too-big fish for your tank, consider exchanging it for a more suitable species in your local fish shop. If that’s not a possibility — and you can’t upgrade its home — empty your tank as much as you can, without stripping it completely bare. Remove decorations to create more room for the fish, but try to maintain its aesthetics as well. Fish appreciate interesting aquascapes too, especially the hiding places.

When populating an aquarium. It is better to have one fish too few than one too many. An overcrowded tank can lead to various problems, not least rapidly rising ammonia levels.

Incompatible Tankmates

When building a communion tank, stay away from overly aggressive species. For instance, there are betta fish that are well-socialized, but there are extremely territorial individuals as well.

And betta fish are only one popular example. There are stranger and subtler exceptions like yo yo loaches. They are peaceful and tranquil if they number three or more. However, a single yo yo is likely to turn its considerable energy toward other tankmates if it is alone and there are no other loaches to bully socialize with.

Lack of Company

Many species are happier when they have proper social lives. The yo yo loaches in the previous example clearly like to have others of their kind to play with.

But shy species like kuhli loach also feel better when not alone. They are not the most social fish at all and even if you have three or four of their kind they would rarely hang out together. However, the presence of other kuhlis gives each individual a greater sense of security. Since there are others like me, then these waters are relatively safe, they must be thinking.

As a result, you will be seeing the shy guys much more often.

Lack of Hiding Places

Everyone likes a place to hide, even the braver species.

That issue is particularly acute with new tanks that are waiting for plants to grow. Many people, and I’ve been guilty of such an approach, prefer to invest in fish rather than plants. After all, plants will grow and reproduce, right?

They likely will, but turning a single cryptocoryne plant into five takes months and weeks with fertilizers. It’s better to put some cash aside to build a well-planted tank from the get-go.

If that’s beyond your budget, consider buying a couple of plants months before you start the actual fish tank and put them in a large jar with a bit of soil. You can grow them on a low budget and produce enough to create a cozy homecoming for the fish.

Driftwood, large stones, and other safe decorations provide an additional sense of security to the tankmates.

Inadequate Nutrition

If they don’t eat well, fish will start losing color and won’t grow. Vary their diet with at least two (preferably three) types of food throughout the week.

Many species appreciate vegetables, so you can treat them to the occasional slice of cucumber or zucchini.

How to Prevent Stress in Fish

  • Maintain stable light and temperature
  • Ensure good filtration
  • Provide adequate food
  • Provide adequate tank size
  • Provide hiding place

Prevention is by far the best way to deal with stress.

Set up your aquarium well, populate it with the appropriate amount and type of fish, provide good nutrition, maintain stable light and temperature, and clean it regularly. These five pillars of fishkeeping are enough to keep your aquatic pets healthy and happy.

If your tank is of adequate size, has good lighting and temperature, there aren’t overly harassing inhabitants and some species still display signs of stress, you must check the water chemistry.

A partial water change done properly can resolve most issues. Regular maintenance is particularly important for new aquariums.  As a watery ecosystem matures, its biodome becomes richer and the water parameters stabilize. But new tanks require attention and regular care.

The most common cause for stress is elevated ammonia levels, but the below advice will help with other issues too.

  • Partial water change — Changing 30%-50% of the water on a regular basis ensures low ammonia and nitrate levels. Naturally, you should test the new water to ensure it has no or low ammonia levels. An ammonia test kit is the most useful tester you can acquire, especially for starting aquariums.
  • Use water conditioner — There are many products that reduce ammonia levels and other harmful elements.
  • Plant live plants — Water plants absorb ammonia, produce oxygen, and provide food and hiding places. Some are better than others at that, but all living plants do it. Oh, and they are beautiful!
  • Increase oxygen levels — Higher oxygen levels don’t affect ammonia or nitrates directly but help stressed fish breathe better.
  • Keep filters clean

What if I Can’t Clean My Aquarium on Time?

If, for whatever reason, you haven’t changed the water in a long while, replacing a large portion of the water column at once might stress the fish.

Abandoned aquariums aren’t a pretty sight. The view of the brackish water and green glass will likely make you want to fix everything at once. But even if the temptation is strong, don’t change 50% of the water. Clean the algae and vacuum but don’t go overboard with the initial water change.

If the fish are alive, then they have adjusted to the poor water conditions.

Better change 25%-30% and then perform another similar change three or four days later, assuming that you have the time for it.

Regardless of the amount of new water, refill the tank as slowly as you can. Either set up a drip system or — often the more practical solution — add the new water in small batches, over the course of a few hours. Pour in a few liters, let the tank rest for 30-60 minutes, repeat.

During the first water change, vacuum the bottom well. The accumulated debris will impact the new water quickly. Remove as much as you can.

Also, don’t forget to measure the water parameters before the first maintenance session, to have a baseline and to understand what you are dealing with. A prolonged period without maintenance may lead to unexpected shifts in the water chemistry dynamics.

That’s why it is a good idea to test the water once a day over the next few days. Always test at the same hour as parameters fluctuate throughout the day. A partial water change will likely address most of the imbalances, but it is best to work with all the data possible.

How to Treat Stress in Fish

Prevention is the best treatment, but sometimes fish will get stressed for no obvious reason. To help overcome it, you must identify the source of stress, and eliminate it. 

Here’s a quick checklist of things to look for:

  • Check water parameters — Ammonia and nitrate levels but also temperature and water flow can affect fish.
  • Observe fish behavior — Is it being harassed by another fish? Is it deteriorating rapidly? Does it look sick?
  • Is there any recent change in the aquarium — New artificial decorations might be unsafe. Even driftwood hides risks if treated poorly. Remove suspicious items to test whether the fish reacts positively.
  • Is there enough room for the fish?
  • Are there enough hiding places?

Once you identify the stressor, correct it as quickly as you can.

Most purely environmental stress factors can be eliminated with a partial water change. That’s assuming that the general water parameters, particularly pH and temperature, are a good match for the fish you buy.

It is a bit more complicated when the fish is harassed by another tankmate. Quarantining one or the other might help, even though the problematic behavior often resumes once both the bully and victim are together.

If the harasser is considerably bigger, you might take it out to a quarantine tank until the attacked fish recovers and grows a bit in size. 

You can also move one of the fish in its own tank for good or exchange it. Local fishkeeping groups or even the fish shop may be willing to do so.


Are stress and disease the same?

Disease and stress aren’t the same. The symptoms could look alike, but the underlying reasons would differ. Excessive, permanent stress will weaken the fish and might make it susceptible to sickness. Conversely, a happy fish in a healthy environment could get sick if a contagious individual is brought in. By definition, a sick fish is a stressed fish.

What’s the difference between shock and stress?

While the symptoms of stress and shock in fish overlap, the latter hits quickly and suddenly, while stress takes hours and days to wear the fish down. Shock is most commonly induced by a drastic, rapid change in temperature or pH, while stress has a variety of causes, as discussed above.

Are fish stressed after a water change?

They may be. Regular water changes inevitably involve intrusion in their habitat. The vacuuming siphon, your hands, occasionally the net to catch larger debris, removing filtering systems for washing, adding new water… You get the picture. Through experience, fishkeepers find ways to minimize stress in their fish during regular maintenance.

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