8 Common Reasons Why Fish Hide Behind Filter (& Solutions!)

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Have you ever noticed your fish constantly hiding behind the filter?

Don’t worry; it’s a common concern among many fishkeepers.

While it’s natural for fish to seek shelter, frequent hiding behind the filter can be due to many other reasons.

Water quality issues, illness, stress, or more…

Ignoring these signs can lead to a decline in your fish health.

Read on till the end to explore the 8 most common reasons why fish stay near filter and how you can fix the issue.

The Normal Fish Behavior?

Depending on the species of the fish you are housing in your tank will depend on the normal parameters of their behavior.

For example, a goldfish will swim around a tank much more leisurely than a tetra that darts from side to side.

a-goldfish-swimming-in-the-tank | why-do-fish-hide-behind-the-filter

Nonetheless, a fish should constantly swim, with a healthy fish only stopping and resting upright near the bottom of the tank to sleep.

If provided with adequate space, food, water parameters, and hiding places, there should be no reason for your fish to hide behind the filter. 

Therefore, if you notice your fish suddenly hiding behind the filter, this should be taken as an indication that your fish is stressed and something is wrong.

Why Fish Hide Behind Filter?

Since hiding behind a water filter is not considered a typical behavior of most fish species, owners should take this behavior as an indication that something in their fish tank is wrong. 

The most common reason a fish may suddenly begin hiding behind a filter is due to the water quality. However, this behavior can also indicate an illness, injury, overcrowding, stress, pregnancy, a lack of stimulation, and more.

Top 8 Reasons For Hiding:


#1. Overcrowded Tank

In the wild, most fish are not restricted to limited areas, and therefore, overcrowding your fish tank can result in aggression among fish, which can further lead to stress and cause your fish to hide behind the filter. 

When fish are overcrowded, some species can turn aggressive and stress out their tankmates, causing them to look for places of shelter, like behind the filter.

#2. Sick Or Illness In Fish

If your fish is injured, they will hide more often. A fragile animal is more likely to be preyed upon in the wild. So, it will do everything it can to hide its misery.

  • Take a thorough look at your fish as it comes out into the open to check for any damage.
  • Your fish may also possibly be unwell in addition to being injured
  • Again, if your fish is sick, they may hide in the tank to avoid being vulnerable. 
  • If your fish is unwell, you’ll need to look for signs of illness. (Not eating and fatigue are two frequent illness symptoms.)

Read Related: Why Is My Fish Swimming In Circles? (Explained)

#3. Poor Water Quality

If your fish is hiding behind the water filter, use an aquarium water test kit to check the water parameters in the tank.

Your fish takes dissolved oxygen from the water in its aquarium to breathe. As a result, the mechanical filtration system agitates the water surrounding the filter output, increasing the surface area accessible for gaseous exchange and increasing the quantity of dissolved oxygen available to your fish.

When your tank is dirty, much of the oxygen produced by the filter system escapes through the water surface, leaving just the region immediately adjacent to the filter with plenty of oxygen for your fish.

#4. New Environment

If your fish is brand new to the tank, they may hide for the whole day until they feel secure. This is a frequent symptom with most fish, and it’s nothing to be concerned about.

There isn’t a remedy for this, unfortunately. All you can do now is attempt to keep the tank peaceful till your fish feels more at ease. It could be a good idea to turn off the lights in the tank and make sure it isn’t being disturbed for a bit.

#5. Fear Due To Aggressive Tank Mates

While certain species are simply timid or shy and will hide whether or not they are agitated, aggression from other members of your tank community is one external stressor that may cause your fish to hide behind the filter in your tank.

The majority of the time, aquarium fish hide when they are disturbed by external circumstances or internal health concerns.

Therefore, if you begin noticing your fish hiding behind the filter in your tank, you should start observing the behavior of the other fish in the tank to ensure your fish is not being bullied. 

#6. Fish Pregnancy

Females who are pregnant will try to hide behind the filter in the tank. This is due to a sense of vulnerability or the fact that they are being bullied by other fish.

It’s quite natural for a pregnant fish to spend more time hiding or looking to investigate the aquarium’s plants and decorations. This is an indication that she is on the verge of giving birth and is seeking a comfortable spot to do it. 

Unfortunately, a female may opt to hide behind the filter in a tank owing to the improved water quality near the filter. This indicates she is just one or two days away from giving birth.

Read Related: At What Time Do Guppies Give Birth?

#7. Lack Of Hiding Places

Especially in a communal tank, hiding places and the tank’s decor become essential in maintaining the health and happiness of your fish.

When a tank is not provided with adequate hiding places, the fish who reside there will look for shelter places, which can include the filter

This, unfortunately, can pose a problem since some filters can suck up the fins of your fish, causing them to get stuck.

Small fish can potentially get pulled into the input of a water filter if the filter is overly powerful. To prevent this problem from occurring, supply your fish with ample hiding spots to keep them away from the filter.

#8. Fish Tank Problems

Fish are sophisticated creatures who require a high level of stimulus to remain happy. Unfortunately, a bored fish may resort to hiding merely because they are miserable.

To boost the amount of stimulation your fish receives, try adding a variety of different toys, plants, and decorations to the tank, or consider keeping your fish company with a couple of tranquil tankmates. 

Although certain fish species have a reputation for being hostile, others get along with most fish species and are typically happier as a result of the companionship.

Do Fish Like to Be Alone?

How likely a fish is to feel lonely mainly depends on their type. Bettas, for example, are solitary in the wild and do not require or desire tankmates when housed in our tanks.

a-betta-fish-swimming-in-the-tank | why-do-fish-hide-behind-the-filter

On the other hand, schooling and social fish like to stay in groups rather than being alone, for instance, neon tetras or cory catfish.

Moreover, a fish can get bored and unhappy if not given enough stimulus.

To learn more, check out my detailed guide about loneliness and boredom in fish.

How To Stop Fish Hiding Behavior?

#1. Correcting Water Parameters

When nitrate levels in the water are high, nitrite in a fish’s blood binds to hemoglobin in the blood cells, impairing its capacity to transport oxygen. 

Your fish may suffocate as a result of this. As a result, the fish migrate to sections of the tank where breathing is less difficult.

To eliminate excess ammonia and nitrites from the water, you may rapidly and efficiently cure the problem by doing multiple modest partial water changes. 

However, don’t do a complete water change at once since this may shock and even kill your fish. Instead, replace 20% of the water and repeat the activity a few hours later. In extreme circumstances, you may need to change the water again the next day.

#2. Treating Injuries or Illnesses

Knowing your fish, including how much they eat, swim, and appear, as well as whether they prefer the surface, middle, or bottom of the tank, will go a long way toward determining if they are sick or not.

Fish, like any other living creature, change their appearance and behavior when they are ill.

Not just for the afflicted fish but for all the fish in your aquarium, recognizing these changes early might be the difference between life and death. 

Once a disease, fungus, or parasite strikes one fish in such a tiny and confined habitat as your aquarium, it may quickly spread through the water to all the others. 

If you believe your fish is sick, remove them from the community tank and take them straight to the vet for diagnosis to prevent further sickness.

#3. Removing Aggressive Fish From The Tank

When a smaller, more aggressive fish bullies it, it will seek hiding spots to avoid unwanted attention. 

Separating aggressive fish from passive fish can assist in reducing this form of hiding. 

Another method is to reorganize the tank’s décor in order to disturb established territories, forcing each fish to re-establish its own niche in the tank.

#4. Addressing Environmental Concerns 

It may seem contradictory, but if your fish tank lacks suitable hiding areas, they may find them more frequently hiding in the same spot. The reason for this is that they do not feel secure. 

Most fish in the wild are prey to larger fish. Thus, they prefer to be in locations with a lot of hiding places.

If your fish feel like they’re too exposed and have nowhere to hide, they’ll return to the same hiding spot repeatedly. 

The apparent solution is to provide them with a lot more hiding places.

Caves, driftwood, and plants will provide your fish with a variety of fascinating hiding spots. These objects will not only protect your fish but also provide them with plenty of areas to explore so they don’t get bored.

FAQs Related To Fish Hiding Behind Filter

Why Is My New Betta Hiding Behind The Filter?

When first introduced to a new tank, bettas may hide behind objects like the filter until they feel comfortable and secure in their new environment. This behavior is normal at first. Ensure proper tank conditions and give them enough time to acclimate in the new tank.

Why Does My Goldfish Stay Under The Filter?

Goldfish staying under the filter could indicate poor water quality, overcrowding, or incompatible tank mates. Test and correct water parameters if needed. Provide more space and hiding spots if overcrowded. Increase surface agitation using an air stone to raise oxygen levels.

Do Fish Like To Hide?

Most fish enjoy having hiding spots in their environment where they can retreat and feel secure. This is a natural behavior. Ensure your aquarium has enough plants, caves, driftwood, etc, for them to hide when needed.

Why Is My Fish Hiding In The Corner?

Hiding in corners or along edges signals that a fish feels stressed or threatened. Causes can include poor water conditions, aggression from tankmates, lack of hiding spots, or inadequate tank size.

Why Is My Fish Always Hiding?

Frequent hiding indicates chronic stress. Likely reasons include bullying tankmates, small tank size, poor water quality, boredom from lack of stimulation, or illness.

Final Thoughts

Since hiding behind a water filter is not an expected behavior among most fish species, owners should interpret this behavior as a sign that something is amiss in their fish tank.

Poor water quality is the most typical cause for a fish to begin hiding behind a filter suddenly.

On the other hand, this behavior might suggest various things, including disease, injury, overpopulation, stress, pregnancy, a lack of stimulation, and more. A healthy fish should swim continuously, pausing only to rest upright towards the bottom of the tank to sleep. 

As a result, if your fish is given enough space, food, water parameters, and hiding spots, there should be no incentive for them to hide behind the filter.

Consequently, if you observe your fish suddenly hiding behind the filter, this should be considered a sign that you need to figure out what’s causing your fish to hide and rectify the situation.

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