Why is My Fish Hiding Behind the Filter?

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Is one of your fish hiding behind the filter lately? This behavior can seem odd, but it’s not uncommon. The filter can be a good refuge for fish thanks to its size and location in the tank. The filter is also a good spot for aquarium fish to hide if you don’t have other decorations for this purpose.

But why would a fish need to hide in the first place? Well, it depends. This behavior could be harmless or a sign of something more serious. It differs on a case-by-case basis. This article will cover the seven most common reasons and ways to identify and treat the underlying issues. Keep reading to learn more!

7 Reasons Why Fish are Hiding Behind the Filter

Hiding now and then is a normal part of fish behavior. If you notice one of your fish hiding, it could just be playing or exploring the tank. However, if the same fish is hiding all the time, this could be a sign of stress, discomfort, or fear.

Many factors can influence your fish’s mood and behavior, so it’s hard to pinpoint the exact reason. You’ll have to monitor your fish closely and tweak things as you go along. Below, you’ll find seven of the most common reasons for fish hiding behind the filter.

– New Environment

Is your hiding fish a newcomer in the tank? Then this behavior is to be expected. A new environment is the most common reason for fish to be hiding. New fish need some time to get accustomed to the water parameters, the aquarium settings, and other fish in the tank.

The fish might seem skittish and reserved during this period. A fish in a new environment might also refuse to eat. There’s not much you can do about this accommodation period except wait it out. After 4-5 days, the newbie fish should start venturing out in the open, explore its surroundings, and interact with other fish.

– Illness or Parasite

You know something’s wrong when a previously energetic and sociable fish suddenly starts hiding. Do all the other fish in the tank seem fine? If yes, this means there’s nothing wrong with the environment. The problem is most likely internal. Perhaps your fish is ill or infected.

Sick fish seek peace and quiet. Besides hiding and avoiding other tankmates, an ill fish will also appear lethargic and have a low appetite. Other symptoms vary depending on the illness or infection. Some other signs to look out for include: swollen abdomen, tattered fins, clamped fins, mucus on the body, inflamed gills, fuzzy spots on the mouth or skin, and golden or white spots on the body.

If you notice any signs of an infection, quarantine the fish in a different tank immediately. Check out some of my other articles about common fish diseases for more help identifying and treating illnesses in fish.

– Aggressive Mates

Fish are sociable creatures that seek interaction. Many fish species prefer living in groups of 5-6 or more. Unfortunately, with socialization, there comes the risk of bullying. Innocent as they seem, fish can be ruthless to one another.

It’s not uncommon for fish to organize themselves in hierarchies. Some unfortunate fish might end up bullied, especially if there are multiple males in the tank. Look for signs of physical injury and notice the behavior of other fish in the tank.

Bullied fish will appear stressed, agitated, and hide a lot. They might also have torn fins, missing or chipped scales, and other physical wounds. Aggressive fish will “patrol” certain areas of the tank and spend a lot of time chasing away other fish.

Once you’ve identified the problem, there are multiple solutions available. Consider moving the bullied fish or the bully to a different tank. You could also add more female fish to the aquarium to decrease male competition. Adding more hiding spaces and spreading the food throughout the tank while feeding your fish can also help.

– Wrong Water Conditions

Water quality is crucial for keeping healthy, happy fish. You might not see poor water quality with the naked eye, but the fish can feel it. Wrong water conditions are physically stressful and can lead to behavioral and physical symptoms.

Pay attention to your hiding fish and other fish in the tank. If poor water quality is the problem, you might notice other symptoms such as fish gathering near water outlets, clamped fins, rapid gill movement, inflamed gills, fish gasping for air, mucus on the body, and lethargy.

Check out the water temperature. The oxygen concentration might drop too low if the water is too warm. Grab your testing kit and check out the ammonia, nitrite, and pH levels too. If anything is out of range, you need to fix the aquarium asap. Ammonia poisoning, in particular, is fatal to fish if left unchecked. The quickest way to get the water chemistry back in check is using alkaline buffers and nitrifying bacteria.

– Too Much Light

Fish don’t need aquarium lights. You can still use lights to beautify your display, but you’ll run into problems keeping the lights on 24/7. Too much light stresses out the fish and messes up their sleeping cycle. If your fish hide behind the filter, they might need shade.

You wouldn’t be able to sleep peacefully with all the lights on either. So, remember to turn off the lights at nighttime. Your fish need at least six hours of darkness to rest properly. Also include some caves and other hiding spots throughout the tank. Sometimes, the fish seek shade during the daytime too.

– About to Give Birth

Hiding is a common behavior in pregnant fish. Female fish will seek out hidden places to give birth in a safe environment. The water quality is also best near the filter, which attracts female fish around this time. And it’s not a behavior exclusive to livebearer fish. Egg-laying fish will also begin nesting when they’re about to lay eggs.

If there aren’t many plants or decorations in the tank, the filter is also a good spot for female fish to get some intimacy. Other signs help you figure out if a fish is about to give birth. She will have an enlarged belly and a bulge at the back of her abdomen. She might also have a gravid spot on her abdomen.

You can help your pregnant fish by providing better hiding spaces in the tank. Alternatively, you can move her to a breeding box. I don’t recommend letting her give birth next to the filter. Small fry might get sucked up the intake pipe if you don’t have a pre-filter sponge.

– Normal Behavior

Does everything else seem in order, but your fish is still hiding? Well, as I said, this behavior can also be normal. Fish have different personalities. Some of them can just be a bit goofy. Your fish might like hanging out near the filter for various reasons.

Hiding near the filter is a way to get food with zero effort. The filter creates a current that draws in food particles. Why bother swimming after the food when it can come straight to you? In other cases, the fish want to enjoy some alone time.

It’s not uncommon for female fish to avoid males. Maybe the female fish is not ready to spawn, or the male fish is just annoying. Either way, fish don’t always seek interaction. Some fish can be more introverted than others. Try adding more decorations in the tank to see if the fish switch hiding spots.

Do Fish Hide When They Are Dying?

It’s common for fish to seek refuge before death. But that doesn’t necessarily mean that hiding is a sign of a fish dying. A hiding fish can be seriously ill or just scared and uncomfortable. Hiding is merely a defense mechanism against environmental and physical stress.

So, don’t panic if your fish is acting weird. You could still do many things to help your pet feel better. Check the water parameters and inspect the fish for physical injuries or signs of illness. If the fish is hurt or sick, move it to a quarantine tank. There are highly effective treatments you can use to treat wounds and common fish diseases.

Common parasitic diseases like ich, fin rot, or velvet can be treated within a few days to a couple of weeks. Most fish medications are readily available in pet stores and online without a vet’s prescription.


Hiding is a normal stress response in fish. If you notice your fish hiding frequently, it could mean that your pet feels agitated, unsafe, or sick. Commonly, a fish will hide because it feels shy or needs some time to rest. A new environment, excessive light exposure, or pregnancy are common and harmless reasons behind this behavior.

In other cases, the hiding fish might suffer from bad water conditions, aggressive tankmates, illness, or parasitic infection. These causes require intervention. Letting them go unchecked would aggravate the problem and potentially kill the fish. If the fish is ill or infected, you also risk the disease spreading to other specimens in the tank.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
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