So your fish has suddenly started hiding behind the filter, and here you are with no clue whatsoever.
A fish may hide behind the filter when it’s threatened, stressed, or feeling uncomfortable. Many reasons that can cause uneasiness in fish include environmental changes, bullying, threats, sickness, injury, pregnancy, loneliness, overpopulation, exposure to intense light or poor water quality.
By identifying the reason your fish is hiding behind the filter, you can take appropriate steps to fix the problem, thus making your fish feel comfortable and confident in the aquarium.
Why Are My Fish Hiding Behind The Filter?
Hiding is an instinctive nature deeply rooted in fish behaviour. In the wild, fish tends to hide to protect themselves from dangers.
Having a safe place to retreat provides a sense of comfort and safety and increases a fish’s survival rate.
While aquarium fish are rarely at risk of being eaten by predators, their self-preservation instincts remain ingrained in their behaviour. As any uncertain situations occur, it triggers the fish’s defence mechanisms spooking them into hiding.
If your aquarium lacks hiding spots or cover, a fish may seek refuge behind the tank’s filter.
As you make your fish feel safe by getting rid of the uncomfortable situation they might have been experiencing, they will voluntarily come out of hiding.
Apart from suspicion of threat, a fish may also hide behind the filter in sickness, pregnancy, loneliness, or other stress-inducing factors discussed below.
1. It’s a New Environment
When a fish is introduced to a new, unfamiliar environment, initially, it will feel nervous and may hide. However, in this case, there is nothing to worry about.
Your fish will come out of hiding as soon as it gets comfortable and feel safe among new tankmates. A fish may take a few days to acclimatize to the new surroundings and aquarium conditions.
2. Tank is Overcrowded
When your aquarium is heavily stocked or too small for all its inhabitants, it causes tension among fish, thus increasing stress levels. In many cases, it may lead to aggression, causing the weak/shy members to hide.
If you believe overcrowding to be the reason for your fish to seek refuge behind the filter, switch to a bigger aquarium or reduce the number of fish.
3. Fish is Being Bullied
Aggressive fish bullying passive species is a common occurrence in aquariums.
To avoid fights or protect themselves, most fish reacts in the same way, which is to seek spots to hide.
Look at the types of fish you have in your tank, and make sure they are compatible with living together. To get rid of fish bullying, you must separate aggressive fish from the peace-loving species or the ones being bullied.
Alternatively, you can try rearranging the tank’s décor or modifying it in a way that disrupts established territories. Doing so will provide an opportunity for every fish to decide on their safe place, allowing them to create a new territory in the aquarium.
4. Fish Feels Threatened
As we know, fish are sensitive beings, any situation that comes off as uncertain is likely to be registered as a threat. At the slightest suspicion, a fish will instinctively retreat to a hiding space.
Apart from being threatened by other residents in the tank, loud noises in the room or movements near the aquarium can make a fish afraid.
5. Sickness or Injury
When a fish is physically hurt or ill, it will seek hiding places to rest and help ease the discomfort. In a condition of weakness, most fish will avoid confrontation with potential predators and bullies.
If it’s been more than 24 hours and your fish is still behind the filter hiding, take a close look at your fish’s appearance, eating habits and swimming behaviour. In case of sickness and injury, fish may act sluggish and lose their appetite.
6. Unfavorable Water Parameters
If your fish is hiding behind the filter or hanging around, it may be due to poor water quality.
Anything that alters the water chemistry, such as sudden changes in pH levels, temperature shift, chemical imbalance, increased ammonia or nitrite levels, and lack of oxygen, are chronic stressors for fish.
Aquarium water, even if it seems crystal clear, doesn’t mean it’s optimal for your fish. The only way to ensure its quality is by testing the water parameters using an aquarium water test kit.
To keep the tank’s water in its best condition, regularly perform partial water changes, clean the filter unit, and replace the damaged or worn-out filter cartridges.
7. Aquarium Light is Too Bright
Bright artificial lighting in the aquariums offers a soothing experience to the human eye, but it’s not the same for your aquatic pets.
Fish are natural habitat to low lighting conditions, so when exposed to high-intensity light, they may try to avoid it by hiding behind the filter. Fish not only dislike bright lights, but it can have adverse effects on their eyesight.
Try turning off the tank’s lights and see if your fish comes out of the hiding space. If it does, take it as a tell-tale sign to replace your aquarium lighting unit with more subdued light.
One way to diffuse the light intensity is by adding floating plants to the aquarium.
8. Fish is Pregnant
Female fish at the time of pregnancy try to avoid unwanted attention from the tankmates and might just go behind the filter. When breeding, it’s quite natural for a pregnant fish to spend more time hiding.
Depending on the species, fish show distinctive signs when they are ready to reproduce. Change in physical appearances, such as swollen belly or developing a “gravid spot” on the abdomen near the rear vent, is the common sign of pregnancy most fish display.
9. Strong Water Currents
Water circulation is crucial in aquariums. However, if your filter, oxygenator or bubbler is generating strong currents or lots of water movement, it can make some fish uncomfortable, especially the smaller ones in the tank.
Try adjusting the flow rate of the equipment to make your fish comfortable so they can come out of hiding. If it’s not possible, add a redirector fitting to help angle the flow, thus preventing the unnerving water movements.
To diffuse the flow rate, you can put rocks, driftwood, or thick planting in the aquarium. Alternatively, you can try covering the end of the outlet using a small piece of sponge.
10. Fish Feels Lonely
Although there is no definite way to know whether fish really get lonely, species that live in larger communities resort to hiding when kept alone with no companions.
Add more compatible species to the tank to ensure your fish do not feel isolated or alone. Some fish are more playful than others and require stimulation to avoid boredom.
Try adding some toys, plants, decor items and perhaps a few more compatible fish if that’s what you lack in your aquarium.
11. Lack of Hiding Places
If your tank does not offer any hiding spots or has few decorations and too much open space, it can make fish insecure, causing them to hide behind the filter unit.
There can be a lot of reasons (as discussed above) for a fish to seek cover. Most of the time, but not all, there is nothing to be alarmed about; it’s simply a fish hiding to relieve stress or feel more comfortable.
If your tank has inadequate hiding spaces, you can add aquarium plants, rocks, ornaments, driftwood or terra cotta pots & PVC piping. It will help your fish feel more secure and at ease, thus encouraging them to swim more freely.
Is It Normal Behaviour for Fish to Hide?
Hiding is an instinct for fish and is completely normal. However, hiding behaviour also indicates that you fish might be stressed due to environmental changes or frightened by bullying or uncertain conditions.
If your fish suddenly starts seeking cover or it’s been too long they haven’t come out of hiding, you must take a close look and enquire about its condition.
Different reasons we have shared above should help you identify the cause or stressor and take preventive measures accordingly.
How long will new fish hide?
Although there is no definite timeline for how long a new fish may hide, the number of days will vary based on how much time fish need to acclimatize with the surroundings.
As soon as the fish starts getting comfortable and feeling secure with its new tankmates, it should come out of hiding.
Do fish hide when they are stressed?
Yes, stress is one the most common reason for a fish to seek cover. In aquariums, there are multiple stressors such as sudden changes in water parameters, bullying, sickness, injury, intense light, uncertain threats, and overpopulation.
Why is my betta fish hiding behind the filter?
Betta fish resort to hiding for the same reasons as any other fish. It’s not about a particular species but a behaviour that is common to pretty much all fish.
How to help a scared fish feel comfortable?
First, identify what is causing your fish to be scared. Once you know the reason behind it, you can take appropriate measures to help the fish calm down.
How to know when a fish is sick?
To determine whether your fish is sick, notice its physical appearance, eating habits and swimming behaviour. Although the signs can be quite distinctive depending on the ailment, here are some commonalities often noticed in sick fish:
It includes changes in colouration, bumps/lumps in the body, misshapen fins, asymmetrical body shape, wounds, loss of appetite, lethargy, partial loss of buoyancy, etc.
And here we are finally at the conclusion of the article. We hope with the information shared above, you now have a firm understanding of the common reasons that cause fish to hide behind the filter.
If you think we have missed out on something that could be a valuable addition to this comprehensive guide, feel free to leave a comment below, letting us know you got something we care about.