Why do Fish Swim More if Water Filter Is Turned Off? [Complete Answer]

You might have noticed that, once you turn off your water filter, your fish become more active. There are a few potential explanations for this and fortunately, there isn’t anything for you to really worry about.

But we’ll get in-depth today and answer all your questions!

Fish might swim more when you turn off the water filter because they don’t have to battle the current caused by the filter anymore.

It’s easier for them to move around when you turn off the filter, as opposed to when it’s turned on – when they might put the same amount of energy, but move less because of the currents it creates.

aquarium filter turned off

Some sources claim that this is simply because they are more relaxed and their energy levels aren’t drained by fighting against the current of the water filter, but there isn’t really any conclusive evidence to back up these claims.

Is it normal for fish to become more active when you turn the filter off?

While this is completely normal, it could also mean that your filter is too strong for the size of your aquarium.

If the current is too strong for the fish to easily move through it, it will cause them extra stress and keep them unhappy. Make sure to always buy a filter properly sized for your tank and adjust its intensity.

It doesn’t have to be like inside a whirlwind there – just enough water movement for the filter to do its job and filter the debris in the aquarium.

But generally, all fish will be a bit more active when the water filter is turned off. Some will even start to swim very fast for a short while after turning off a water filter because they sense the change in water currents and don’t know what to make of it.

Related reading: Why are fish more active when your turn off the lights?

Why do fish keep swimming against the filter?

small filtered aquarium

If your fish are constantly swimming against the current of the filter, it could mean that they are not getting enough oxygen otherwise.

Fish will also try their best to avoid areas with high concentrations of nitrates and ammonia, and the filtered water might be the best place in the aquarium for it.

So if you notice that your fish keep swimming against the filter, check your water parameters. If not possible, do a partial water change by changing 25% of the water in the tank.

If, after you do this, they no longer swim against the current, it means that your water parameters were off and you should keep an eye on them.

But this doesn’t always signal bad things. Sometimes I see my smaller fish swimming against the current for a while, before retreating to an area where the current is not as intense. Maybe it’s just a natural thing for some species to do.

However, never guess that everything is good! Better be safe than sorry – so do a water test or change the water, just to stay on the safe side!

Why do fish hang near the filter when it is turned on?

If your fish group together close to the filter when it is turned on, it might mean that the filter is too strong for the aquarium.

The current is the least intense in the proximity of the filter (and strongest in the opposite part), so fish will simply sit where the water is calmer, because it’s too difficultt o swin otherwise.

This is again a reason for your fish to become more active once you turn off the filter – since the current is not as intense, they can now swim freely throughout the aquarium.

Some other fishkeepers believe that this behavior is caused by the increased oxygen concentration near the filter, but it’s usually a too strong filter the reason why they all group up close to the filter.

Check out my recommended aquarium filters for large tanks to make sure that you make the best choices when it comes to picking the right one for your aquarium.

aquarium filter in my fish tank

Can a too strong aquarium filter kill your pet fish?

Yes, a filter that is too strong can actually kill your fish. If you have a very powerful pump in your tank, then this could be detrimental to your fish. It will cause them stress, which can lead to health issues and death.

A really strong filter could also cause direct damage to your fish – especially to their fins which are an essential part of their body – as well as prevent them from sleeping or resting.

These all will eventually weaken their immune system and harm them. This is why you should always make sure that the filter you have installed is the best possible for the size of your tank.

Should you turn off the filter in your tank every now and then?

It is an ongoing debate whether the filter in your aquarium should be turned off every now and then.

If your filter is not too strong for the aquarium, then there is no real need to turn it off. In fact, it could cause more harm than good if you turn off the filter too often or for longer periods of time.

If your tank is not extremely well established, or maybe a bit overpopulated, then turning off and on the filtration system every now and again might cause harm, allowing a buildup of harmful substances in the aquarium.

On the other hand, if you have a well-established, stable aquarium with little waste production, then it is possible to turn the filter off for 8 hours every now and again.

However, what usually happens when people do this is that they end up turning on their pumps too early or leave them on for too long after having turned them back on because of visible algae in the aquarium.

In my case, since I am all about creating self-sustaining environments, I am testing a filter-less tank for a few months now and things are looking good.

However, I still turn the filter on for about 4-5 hours each day, perform constant water changes and I also have a huge aquarium with tons of plants and very few inhabitants (30-gallon, heavily planted tank with 12 microrasboras and 6 neon tetras).

If you have more fish in your tank, producing more waste than it can naturally be taken care of, then it’s better to leave the filter on at all times.

Just make sure that the filter is properly sized for your tank (and not too strong) and things will be OK.

You can learn more about how a filter works in a fish tank here.


I ended up answering quite a few questions regarding filters and fish behavior, and not just why fish become more active when the water filter is turned off. Hopefully, this was helpful to you.

If you have more questions regarding filters or other stuff related to aquariums and fishes, feel free to leave me a message.

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