Eric is an aquarium enthusiast with over two decades of experience caring for a wide array of tropical fish.
How do you recognize stress in fish?
Fish exhibit a range of behaviors that indicate their mood and health. How they swim, react to other fish, and behave around food can tell you a lot, as can their physical appearance. There are many specific things to watch out for, and you’ll learn about them in this article.
One of my top pieces of advice when it comes to fish care is to make your aquarium as stress-free as possible. That means choosing the right tank, stocking compatible fish, and learning how to manage your aquarium.
But we all make mistakes, and when things go wrong in your tank, your fish will be the first to let you know. Sometimes the cause isn’t so obvious, though.
So, how can you tell if a fish is stressed?
10 Signs of Stress in Fish
Here is a list of the ten most common signs of stress for aquarium fish.
- Glass surfing
- Flitting around the tank
- Excessive hiding
- Changes in appearance
- Constantly chasing other fish
- Fin deterioration
- Loss of appetite
- Rubbing against gravel or decorations
- Gasping near the water surface
- Persistent disease
Here is a closer look at each of these issues, one at a time.
1. Glass Surfing
Unlike in their wild habitats, the fish in your tank have nowhere to go when they are unhappy. They can’t leave when water conditions are poor or when another fish is giving them grief.
When some fish are stressed, they exhibit a behavior known as glass surfing. This is when a fish swims up and down the sides of the tank glass for hours on end. It is restless, and it wants to get the heck out of there.
Almost anything that causes stress can lead to glass surfing, so consider it a general sign that something is wrong in the tank. To figure out what is going on, you will have to do a little investigation. Consider all the causes of stress listed near the end of this article and rule them out one by one.
2. Flitting Around the Tank
Startled fish can injure themselves by running into aquarium decorations or glass. Some fish will even jump right out of the tank when they get scared. There a couple of reasons this might happen.
Some fish are just skittish. This is especially true of schooling fish, and if you stock them you need to think about the kinds of things that might freak them out. Big fish that can eat them may scare them, as might fast-moving fish that dart around the tank.
It is important to stock fish that are compatible, not only so they don’t harm each other but also so they don’t stress each other out.
Another cause might be you! Did you place your aquarium where there is a lot of human activity? Are people always pressing their noses up against the glass or tapping on it? These things can create stressed fish that are always on the verge of fight or flight.
3. Excessive Hiding
It is normal for fish to seek places in the tank where they can feel safe. Some fish species such as catfish hide more than others, and some hide at certain times of the day or night.
However, if you notice a fish you wouldn’t expect to hide tucked away in a corner of your tank all the time, this is a good sign that something is wrong.
Stressed fish hide when they feel threatened. This might be because of aggressive tank mates, or it might be because they share a tank with another dominant member of their species. Watch for other fish chasing them when they come out of hiding and you might better understand the problem.
Fish may also hide because they are injured. Again, this could be because of an aggressive tank mate, but it also could be because they scraped against a sharp decoration or another object in your tank.
4. Changes in Appearance
Extreme stress can have a terrible effect on fish. Those that are bullied or harassed by others in the tank can experience physical changes. Fish don’t have a lot of weight to lose, but when under constant duress they may look as though they are wasting away.
It’s awful, and if you have ever seen this happen to a fish, you will never forget it. Obviously, the result is premature death.
Colorful fish can also lose their luster when stressed. Bright fish become duller. With betta fish, their faces and jaws turn a light grayish color when they are stressed.
5. Constantly Chasing Other Fish
Stress goes both ways in a fish tank. When fish are bullied, it is very stressful for them, but it is stressful for the bully, too.
Some fish are considered semi-aggressive. They may be territorial, and some just don’t like smaller, quicker fish darting around them all the time. Some don’t like others of the same sex. Some don’t like others of the same species.
Whatever the reason, if you stock them with fish that stress them out, you may see them chasing the other fish away all the time.
That situation is stressful for everyone. In fact, it is even stressful for fish in the tank that have nothing to do with the interaction. Aquariums should be calm, peaceful habitats, not battlegrounds.
This is why it is so important that you research and understand the needs of every fish you stock.
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6. Fin Deterioration
Fin rot is an affliction where a fish’s fins gradually deteriorate. In the worst cases, it can spread to the body of the fish. It’s a bacterial infection, and it is nasty, but it is also very preventable.
Poor water conditions are often to blame, especially when fit rot affects betta fish who live in small tanks. Their tanks become polluted, the infection sets in, and their fins deteriorate. This kicks off a kind of stress cascade where it becomes harder for them to recover, even after you correct the issues that led to the fin rot.
Other kinds of fish can suffer from fin rot, and stressors in the fish tank usually bring it on. Overcrowding, fin nipping, and changes in the environment are all potential causes.
7. Loss of Appetite
I sometimes get questions from readers who are panicked because their new fish isn’t eating. Usually, I tell them to relax and give it time. Moving into a new tank is stressful for fish. Sometimes they take a little while to calm down and make themselves at home.
It is a different story if an otherwise healthy fish stops eating. While all fish may occasionally turn their noses up at perfectly good flakes, they need to eat eventually. The first step is to try some different foods just to see if they might react differently.
After a few days, it is time to wonder if something is wrong. Illness, bullying, and environmental conditions are all plausible explanations.
8. Rubbing Against Gravel or Decorations
Scratching and rubbing on rocks, gravel, plants, and decorations are a sign that something isn’t right in your tank. Poor water conditions with high levels of waste chemicals are often the problem.
You can’t guess whether your aquarium is safe. It is smart to use a freshwater testing kit to measure ammonia, nitrate, nitrites, and pH. This is the only way to have an actual picture of what is going on with your tank water. It is also an excellent idea to have your water sources tested.
One more reason fish may rub against decorations has to do with a parasitic infection called Ichthyophthirius multifiliis, more commonly known as ich. Look for little white spots on your fish.
9. Gasping Near the Water Surface
When tanks are low in oxygen, fish may come to the water surface to breathe more easily. Some new aquarists think adding an air stone is the answer. Aeration is important, but a good filter should move enough water to do the job.
There are other things to think about. Water temperature, tank overcrowding, and water quality all play a role. Checking through these issues one by one, starting with testing your water, might help you understand things.
It is important to note that it is normal for some fish called anabantids to breathe air at the water surface. The most common anabantids in the aquarium industry are betta fish and gouramis. Still, in a healthy aquarium, they should get all the oxygen they need from the water.
10. Constant Disease
Your fish shouldn’t be sick all the time, and if they are something is wrong. Many diseases are brought on by stress or made worse by stress. Even parasites like ich have an easier time spreading when fish are stressed and weak.
Maintaining a stress-free, healthy tank is the most important thing you can do to ward off afflictions like dropsy, fin rot, and popeye. Yes, your fish might still get sick from time to time, but far less often. And, when they do, they have a better chance of recovery.
Causes of Stress for Fish
If your fish exhibit any of the above behaviors it can be tough to figure out what to do about it. It often takes a little investigation to figure out what is stressing your fish.
Some of the reasons fish experience stress include:
Tanks that are too small don’t give your fish enough room. How much space do they need? There is no hard-and-fast rule that applies to every fish species. It is so important to research fish before you bring them home, so you know they are right for your tank.
Poor Water Conditions
It is vital that you stay on top of water changes and regular tank cleanings. If you neglect these simple maintenance tasks, water can become toxic to your fish. This leads to stress, disease, and early death.
Overstocked tanks are a source of stress for every fish and critter inside. Not only do they have no room to swim without invading each other’s space, but crowded tanks are often heavily polluted.
Aggressive Tank Mates
You must choose fish that can get along in the same tank. The only way you can know if you are stocking the right fish is to do the research before you bring them home. Yes, I know I've said that a lot in this article. It is extremely important.
Amorous Tank Mates
Sometimes tank mates are a little too friendly! This often occurs with livebearers like guppies and swordtails. Males looking for love relentlessly harass the females to the point of stressing them out. Many fishkeepers solve this problem by stocking livebearers at a three-females-for-every-one-male ratio.
Lack of Similar Tank Mates
It is best to keep schooling and shoaling fish in groups of six or more. Any fewer and they become stressed. This is a common mistake made by many new aquarium owners.
Poorly Chosen Tank Locations
Aquariums should be in low-traffic areas, out of direct sunlight. Excess sunlight can lead to algae growth, and your tank conditions can spiral out of control in a hurry.
Tapping on the glass and constantly trying to interact with your fish only stresses them out. Let them do their thing in their little world and observe them from a respectful distance.
Fish sometimes get sick because of stress, but it is also true that sick fish suffer from stress. It’s a feedback loop you want to avoid by making sure your aquarium inhabitants live healthy, low-stress lives.
Mitigating Stress in Your Aquarium
Managing stress is the most important thing you can do for your fish. If you can do this, you’ll also be taking care of all the other things they need.
In this article, you learned about 10 signs of stress in fish, as well as several causes of stress. You may have noticed it all boils down to a few important points.
Clean water, the right tank mates, a big enough tank in a calm, peaceful location is what it takes to help your fish stay stress-free.
This article is accurate and true to the best of the author’s knowledge. It is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from a veterinary medical professional. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.