Fish Staying Near Filter (Explained)

Fish Staying Near Filter

Fish in overcrowded tanks tend to hide. When you notice you have fish who are always by the filter check to see if other fish are hiding in the decorations because it may be time for a larger tank.

Although many fish swim in schools, not all species do, and some fish make bad neighbors. Why is your fish staying by the filter?

Fish sit under the filter because they are looking for a place to hide quickly in an overcrowded tank. When there’s nowhere else to go, frightened fish will retreat anywhere they can to avoid confrontation with unpleasant tank mates. This innovative survival skill keeps them out of fights and can even save a fish from getting noticed and eaten.

Why Does My Fish Sit Under The Filter

1 – Overpopulation & Hiding

Too many fish in a tank will send some running. Of course, some species are natural enemies or just more aggressive in general.

Bad tank mates and not enough places to swim will cause some fish to sit under the filter because the outgoing current is uncomfortable to certain other fish. If you have a mean or predatory fish, it’s best to house them alone, but the rest need enough space.

How many fish are too many for the tank? The answer depends on the tank size, plus fish species and size.

However, according to Wikihow’s pet section, “The general rule of thumb is 1 inch of fish per 4 liters, so you could have around five 3 inch fish, or four 4 inch fish.”

2 – Instincts & Feeding

Some fish sit near the filter out of instinct. Just as there are fish who prefer to stay out of the current, others seem to enjoy the feeling and seek it out.

Moving water tends to be clean and easy to breathe in, but there’s another advantage to hanging out by the filter. Fish can be lazy.

The moving water will pick up fish food particles or smaller fish and push them right into the waiting jaws of larger fish waiting nearby.

As opportunistic eaters, this is the closest thing they’ll ever get to a drive-through or delivery meal. Easy access to snacks is a significant bonus for hungry fish.

3 – Cleaning & Breathing

The filter in your fish tank is living up to its name by removing debris and waste. Moreover, the filter helps aerate the water, so it’s got plenty of oxygen for the fish to breathe.

Some fish hang out by the filter because their tank isn’t clean enough or is low on oxygen.

Why Does My Betta Fish Stay Near The Filter

Betta fish stay near the filter for entertainment. These fish are more than just pretty colors; they are smart, at least by fishy standards.

Boredom is the natural enemy of betta fish, and they can even become depressed and lethargic without something to do.

A depressed betta will become dull in color, stop swimming around so much, and may even stop eating normally, which is a bad sign.

Your bettas need more than a place to exist. The fish are in a direct current by hanging out near the filter, which they enjoy as long as it’s not too strong.

Doing this gives them a challenge to swim against. Some fish hid behind or under a filter to escape doing exercise or to get more food faster, but not this species.

It’s a form of exercise and entertainment for a betta fish to hang out near the filter where there’s a good current.

Why Does My Goldfish Stay Under The Filter

Goldfish often hide under the filter when they are pregnant. A gravid fish is at risk because they are slower and weaker than average, which leaves them open to harassment.

For a pregnant goldfish, below the filter is often a place to hide or find some peace away from the other fish.

Even if you don’t have other fish, a goldfish may still have this odd quirk while pregnant. In this case, it’s not about actual bullying but rather an instinct to protect herself and her future young.

According to Aquarium Nexus, “There is no definite time that the goldfish stays pregnant, but they carry the eggs until they are fertilized by the male. After release and fertilization, goldfish eggs hatch within two to seven days with the right water conditions and requirements.”

If your female goldfish lives alone, this is unlikely to be the case, and you may need to clean her tank.

Pregnant goldfish are skittish, but non-pregnant members of this species are as likely to be hunting for air, clean water, or food as any other fish.

Molly Fish Staying Near Filter

Molly fish that are staying near the filter are trying to tell you that there’s a problem. From ammonia buildup to low oxygen and generally dirty conditions, these sensitive fish like a good, clean tank with lots of oxygen. You should check your tank.

Changing the water and regular cleaning are always important. However, Molly’s are often among the first to ‘complain’ if the balance is off.

Even if you have automatic systems in place, they can fail, so if you see a Molly by the filter, you need to check everything from pH levels to temperature and filter function right away.

Helpful Tips To Know About Fish Staying Near Filter

Some fish are just having fun or eating snacks. You don’t always need to worry about fish staying near the filter, but it’s a good idea to check the water for air, cleanliness, and the correct temperature just in case.

Here are more helpful tips to know about fish staying near filters.

  • Female fish may hideout near the filter together during breeding time. If a group of girls are there, then they are working as a group to stay safe. Doing this makes them feel secure, and they can fend off unwanted advances or help keep all the eggs safe.
  • One of the simplest ways to spot a dirty tank is to look at the bottom. If there’s a layer of uneaten food and debris, it’s past time to change the water and clean your tank.
  • It is essential to make sure your fish are either the same species or at least compatible before you put them in a tank together. Hiding near the filter can’t stop a bully forever, and it certainly won’t prevent fish from getting eaten by a determined predator.

Final Thoughts

Nervous and frightened fish hide behind filters, and so do pregnant fish who want to stay safe. An overcrowded tank or mean neighbor can run a fishes’ day.

However, there are other reasons a fish will go hang out near the filter. Bettas like the current and take it as a personal challenge to swim there for stimulation.

It’s a form of play for them. Meanwhile Mollys and other fish may be near a filter because they can’t catch a breath, or the tank is too dirty.

When in doubt, always check the tank first to ensure the water is clean, aerated, and at the right temperature, then make sure your fish have plenty of space and features to explore and hide inside. Happy fish are healthier overall.

Ted Smith

My name is Ted Smith and I’m the creator of I have a passion for educating people about animals and wildlife. I have been working with the National Wildlife Federation for the past 10 years and I became a wildlife blogger to help people become excited about animals and encouraged to care for these wonderful creatures.

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