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Freshwater Fish Abnormal Behaviors

All fish types (both freshwater and salt water) have similar normal behaviors. But do they have similar abnormal behaviors? “Freshwater fish abnormal behaviors” seeks to give more light and explanation on that.

Fish are not like other pets such as cats, dogs, or even lizards that you can handle and spend time with.

They need to remain inside their tank (aquarium) always. Thus, to fully know and understand your fish, you need to observe them and learn their behavior keenly.

Understanding your fish behavior is critical to know when they are sick, happy, or stressed.

Some behaviors show their contentment, while others their distress. When they are happy, their behavior is expected to be normal ranging from normal swimming patterns and regular eating habits.

However, when your freshwater fish aren’t happy, that is when they’re sick, you’ll notice behaviors that devoid normal behaviors. These are called abnormal behaviors which this article will look at all of them. Most people cannot differentiate their behaviors.

As already stated, this article is to help you learn/understand your freshwater fish abnormal behaviors.

Keep reading!

How to Tell If Your Fish Is Unhappy?

signs of a happy and healthy freshwater fish
You can tell your freshwater fish is happy if it has clear eyes, smooth scales, swimming up and down and have its fins widely opened while swimming.

Are you afraid that your fish are unhappy? It can be challenging to tell when your fish are happy or sad, especially for beginners.

You can tell your freshwater fish is happy if it has clear eyes, smooth scales, swimming up and down and have its fins widely opened while swimming.

Fish become unhappy when they are stressed or sick. And, stress and illnesses can quickly kill your fish if left unattended.

Therefore, it is inevitable for every aquarist to analyze their fish behaviors. Consistently watching your fishes for at least 10 minutes a day can help you know their normal behaviors.

That way, you can quickly notice any behavioral change amongst your fish. Change of behavior indicates abnormality and can be dangerous. 

Therefore, it is your responsibility as a fish hobbyist to keep your fish happy and healthy. 

There are multiple reasons why fish become unhappy or stressed in the aquarium. They include:

  • Lack of adequate oxygen
  • Overcrowding 
  • Inadequate or excessive temperature
  • Harassment from other fishes
  • Poor nutrition
  • High nitrate levels
  • Improper PH in the tank
  • Lack of hiding places

Freshwater Abnormal Behaviors

Any behavior that is not within the usual norms in the aquarium is abnormal. Since fish cannot speak or show emotions, we know they are sick or stressed by observing their behavior.

A sudden change of behavior can be an indication of stress or illnesses. In this section, we will list freshwater abnormal behaviors. 

freshwater fish eating
This might look like gasping, but the fish is attracted to the food it wants to eat

Loss of Appetite

Fishes have a large appetite, and they enthusiastically swim towards the food during feeding time. 

However, some are picky and may refuse a particular diet. Even so, if you know your fish well, you should be in a position to distinguish between a normal and abnormal loss of appetite.

Loss of appetite is a common symptom in most fish diseases. For instance, fish suffering from dropsy, fin rot, mouth fungus, and swim bladder may experience acute loss of appetite.

Additionally, inadequate temperature, bad water quality, and any other stressor can affect your fish’s appetite.

Therefore, it is critical to identify any possible cause of loss of appetite in your fish and quickly eliminate them.

Abnormal Swimming Behavior

Every enthusiastic fish hobbyist spends some time watching their fish swim in their tank every day. 

Therefore, they know how their fish swim and can tell any slight change in their swimming behavior.

Change in swimming behavior can be a dangerous sign. For instance, fish suffering from swim bladder swim upside down at the top of the tank.

Fish suffering from fin injuries or fin rot may also have difficulty in swimming. Therefore, you should physically examine your fish in case of any change in their swimming behavior.

Additionally, you should consult a vet for proper diagnosis and enhance your fish’s habitation to keep them free from stress.

Erratic or Fast Swimming

Your fish may suddenly start to swim erratically during breeding or in case of a drop in water quality.

Freshwater fishes are sensitive and quickly react to any slight change in their environment. 

Therefore, erratic swimming should put you on toes and immediately test your fish’s water quality and adjust where necessary.

But, if everything seems to be on point, then it could be that your fish are breeding.

In some species, such as loaches, the male chases the female around the tank so fast until they release the eggs. The male then releases sperm to fertilize the dropped eggs.

Hovering At the Top of the Tank

Hovering At The Top Of The Tank
This behavior depending on the species of fish can be normal. However if it’s a repeated behavior especially if your fish don’t normally swim or float at the top of the tank, this can be abnormal.

Some species spend their time at the top of the tank, others at the mid-level, and others at the bottom.

If you know yours are mid-level or bottom-dwelling fish and they suddenly start staying at the top, it can be problematic.

Fish suffering from inadequate oxygen swim at the top of the tank, where the oxygen is plenty.

Therefore, if your fish are suddenly spending the most time at the top of their tank, you should immediately check the oxygen levels in their tank.

You should also immediately change their water and add chemical and mechanical filtration to keep the water clean and clear.

Additionally, live plants may also help to add oxygen levels to your fish aquarium.

Fights amongst the Mates

Some fish species are aggressive and territorial. Thus, they tend to fight other species in an attempt to claim their territory.

In most cases, fights occur when you mix several species in the same tank. Also, overcrowded fishes may start a territorial battle to claim their space.

Therefore, it is critical to ensure that you provide a large tank, especially considering a community tank.

Adding more live plants in your aquarium can also help to reduce territorial fights. 

Rearranging Tank Objects

Rearranging tank objects may be typical to some species especially, bottom feeders. 

Bottom feeders such as loaches dig into the substrate where they get their food. Therefore, it is normal behavior for them that should not alert you.

However, sudden digging and rearranging behavior can be a stress indicator in non-bottom feeder fish.

Therefore, if your fish suddenly starts rearranging their tank, you should look for the possible reason and adjust ASAP.


It is usual for nocturnal fishes to spend their time hiding during the day. But, if your fishes are diurnal and rarely shy, they should spend their day swimming actively.

A sudden hiding behavior can be a sign of aggressiveness from other fishes. Therefore, you should ensure to keenly observe, eliminate or separate any aggressive fish from the rest.

Aggressive fish can cause secondary infections to the victims, such as fin rot. Therefore, it is imperative to separate any aggressive fish from the others.

For any kinds of illness or disease you suspect your freshwater fish to be having, Melafix Fish Remedy or it's equivalent is the best product you can use to treat your fish at home before seeing a vet.

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Fin-nipping may be normal behavior for some species especially, in a community tank. 

However, it becomes a problem when the nipping is continuously repeated. It can cause permanent fin damage or lead to secondary infections.

Therefore, please don’t mix fin-nipping species with fishes with long fins.

Additionally, you should ensure to keep fin-nipping species in a large school to discourage the behavior. 

Weak Movement or Lethargic

If your active fish suddenly becomes lazy, you should get worried as it can be a sign of illness or stress. 

In most cases, bad water quality, low or high temperature, ammonia poisoning, or wrong PH balance can cause your fish to become lethargic.

Therefore, you should test your fish’s water quality as soon as you notice the behavior.

Additionally, parasitic infections and a bad diet can also cause lethargy in freshwater fishes.

Therefore, you should also consider those possibilities and treat them immediately.

Rapid Gill Movement 

Your fish should have a normal gill movement to show they are not straining to breathe.

However, if you notice a sudden change in gill movement in your fish, it can be due to stress or sickness.

Bad water quality can cause rapid movement, and so can infections such as fluke, ich, or mites.

Prolonged stress in the aquarium weakens your fish immunity. As a result, your fish become susceptible to fungal, bacterial, protozoan, and parasite infections.

Therefore, it is necessary to provide a stress-free environment for your fish to avoid these problems.

Sitting At the Bottom of the Tank

freshwater fish abnormal behaviors
A freshwater fish resting/sitting motionless at the bottom of the tank

Bottom feeder fishes spend time at the bottom of the tank while searching for food. 

However, if your fish are mid-level or top feeders, sudden changes such as staying longer at the bottom of the tank can be an alarming sign.

It can be a sign of sickness, such as a swim bladder. The swim bladder is a dangerous condition that can kill your fish if not adequately treated.

Overfeeding is the primary cause of the swim bladder. Therefore, if you notice that your fish seem to be staying longer at the bottom, you should look for other signs such as lethargy.

Also, you should test the water quality and starve your fish for at least three days. On the fourth day, their diet should include frozen peas as it helps to treat swim bladder.


Flashing is when your fish rub against objects that may be due to skin irritation. Flashing is abnormal behavior and never to be taken lightly.

The primary cause for flashing may be parasites such as ich or fish lice. Flashing can lead to bruising, missing scales, or traumatic wounds. 

Additionally, skin erosion due to flashing can cause secondary infections such as mouth fungus or fin rot.

Therefore, you should look for the underlying causes of flashing and treat them immediately.

Gasping for the Air

A freshwater fish fish gasping for air
A freshwater fish fish gasping for air

Fish should have normal breathing. Sudden gasping of the air is dangerous. In most cases, fish gasp for the air when there is insufficient oxygen in their tank.

Low oxygen levels in the aquarium come from bad water quality, especially when there is excessive nitrate.

As a responsible aquarist, you should test the water quality every day. Also, you should frequently change the water and ensure that the filtration is working correctly.

Lack of sufficient oxygen in an aquarium can put your fish in great danger. It is because it causes acute stress that leads to low immunity in fish.

Fish with low immunity get exposed to viral, fungal, bacterial, and protozoan infections.

Therefore, it is imperative to maintain good water quality to supply adequate oxygen in your fish’s aquarium.

Sudden Aggression

Freshwater fish such as goldfish are naturally very docile. A sudden change of behavior such as being aggressive towards others is abnormal behavior.

Sudden aggression in docile fishes can be a sign of acute stress. When there is not enough food for the fish, they might become aggressive when fighting for the little available.

Sickness can also cause stress on fish. In return, they become aggressive and can seriously injure others.

Therefore, if one of your docile fish suddenly becomes aggressive, immediately quarantine it and look for the underlying reasons.

Freezing Behavior

Freezing behavior is typical among prey fishes. It is a response behavior on how fish react to specific stimuli. 

However, it can cause rapid breathing, choking sensation, or increased heartbeat in the affected fish.

In most cases, it happens before, during, or after a fight. When a prey fish is caught and defeated by the predator, it may freeze due to blood pressure change.

Freezing can cause acute stress for your pet fish. Therefore, you should avoid mixing prey and predatory fishes in the same aquarium to discourage freezing.


Betta fish female’s gills may turn dark to attract the male. However, pale gills can also signify sickness in other species, such as koi and goldfish.

Datylogyrus and Neobenedinia are infections that affect koi and goldfish, which are freshwater fishes.

It causes their gills to turn pale and swollen. Therefore, discoloring especially, turning pale can be abnormal.

Being Picky On Food

If you are keenly observant of your fish’s eating behaviors, you may notice your fish being picky on some food.

For instance, betta fish hobbyists say that their fish eat red flakes and leave out other colors. 

Most aquatic hobbyists perceive this behavior to be abnormal or an eating disorder. However, it can be that the red flakes are more visible to them, and they are just eating what they see.

Also, it could be that the red flakes have a better taste. The latter being more realistic as some hobbyists confess their fish spit out another color in the sight of the red ones.

Some common freshwater fish abnormal behaviors, causes and treatments. [source: Master Aquatics]

Eating a Whole Worm and then Spitting.

Betta fish owners complain of their fish eating an inch-long worm and then spitting it out and then taking them back instantly.

Most hobbyists perceive it to be abnormal. However, it can be normal behavior for betta fishes as they have teeth on the roof of their mouth to the throat.

Therefore, eating a whole worm, spitting, and taking it back in the mouth can be their form of chewing.

Even though the behavior looks abnormal to the most hobbyist, it makes sense considering their mouth and intestinal structure.

 Eating Dead Snails

Some freshwater species such as senegalus tend to like dead snails. They may completely ignore live snails and go crazy on the dead ones.

But, don’t get overworked by this behavior; polypterus senegalus consumes any edible thing, whether dead or alive.

Thus, whether they love dead or live snails should not worry you at all. Just feed them according to their preferences as long as the food is clean and nutritious.

Gluttonous Behavior

 Some fish species, such as guppies, are typically gluttons. They spend most of their time foraging, and they can overeat if you don’t limit their food.

The best way to feed guppies is by offering them just what they need. A portion of food they can eat in five minutes should be enough.

Feed them once or twice a day for the adults and up to five times for guppy fry. Additionally, you can occasionally skip a day without feeding them if they seem to be overfeeding.

Overfeeding can cause health issues to all types of fish. Overfeeding can also be a sign of stress in other species.

Therefore, if your fish just started to be gluttony, check out any possible stress factors in their habitat.

Regular change of water and proper filtration can save your fish from lousy water quality stress.

Playing Dead

Some fish species may act dead to fool other fish, preferably targeting scavengers. However, in other species, it can be abnormal behavior. 

If your fish seem to be floating at the top of the tank upside down, it’s not acting dead. Instead, they could be suffering from dropsy or swim bladder, which is a common illness in most species.

Therefore, if your fish seem to be acting dead, immediately examine them to see if they have other symptoms portraying illness.

Other common symptoms of swim bladder and dropsy may include a swollen belly, loss of appetite, sunken eyes, and floating at the tank top. These illnesses can be fatal if left unattended.

Wrapping Up

Every dedicated fish hobbyist must spend quality time learning their fish behavior. It is the only way they can know when their fish are in trouble and need help.

Fish are non-sentimental. Thus, it is impossible to know when they are stressed, happy, or ill without understanding their typical behavior.

Any slight change in your fish habitat can magnificently cause a behavior change. 

Stress and illness affect your fish swimming, eating, and social behavior. Therefore, every hobbyist must get familiar with their fish’s typical behavior to keep them happy and healthy.

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