Well as you are in the fish keeping hobby, I assume you already know about nitrogen cycle and aquarium filtration. It is the basic of fish keeping and mandatory as well to keep your aquatic pets alive and healthy.

In aquarium filtration, the tank water is passed through the filter media and as a result debris, toxic chemicals, uneaten foods and other wastes are removed from the water column.

Rule of Thumb For Filter Selection

When suggesting a filter, we usually recommend a filter with about 6x turnover rate of your tank’s water volume.What does that mean? It means you should select a filter than can cycle or pass through the water of your whole tank at least six times in every one hour. For turtles or chiclids the recommendation is 10x filtration.

The target is to keep the ammonia and nitrite level zero all the time. A sudden spike in ammonia or nitrite would be deadly for your fish.

What is Over Filtration? Is it Bad?

Now if you select a filter that has way more capacity than the recommended 6x volume we can say it over filtration. Seems like a good plan, huh…?

Now comes the interesting question.

Is over-filtration bad for you tank?

Not necessarily! Rather it depends on how you are doing it! Keep reading to know more about it.

What Happens If You Select a Very Large Filter For A Small Aquarium

Lets say you have a 40 gallon tank. You are using a 880 lph (liter per hour) Top Filter in there, which is perfect for your tank. Now If you use a 1,500 lph Top FIlter instead, the beneficial bacteria gets very less contact time with the water and the biological filtration becomes ineffective.


Beneficial bacteria grow inside the filter media and consumes harmful elements like ammonia and nitrite. To work (or should I say consume) properly, the bacteria should come in contact with the water for certain period of time. If the filter is too powerful compared to the tank volume then the bacteria will simply not get enough contact period.

That will result in inefficient filtration even though you are using a higher capacity filter.

But There is a Myth in the Aquarium Hobby

You must have heard it! The myth is – “There is no such thing as over filtration”. Well, it is partially true and if you do things properly you can take the benefit of over filtration as well.

How To Do Over filtration Properly

So how do you over filter a tank?

By using two filters!

Instead of one large filter, which is say 16x of your tank capacity, if you use two 6x capacity filters then each filter will turn over the tank about 6x per hour. And that is how you over filter a tank.

Benefits of Over filtration

Why would you want to over filter a tank?

The answer is simple – there is no such thing as too much filtration (hahaha..), but it depends on how you are doing it, as mentioned before.

Not only would you have pristine water conditions, you also have redundancy. In case one filter fails, there is always another one providing vital life support to your tank.

This method is specially applicable for tanks with large fish or high bio-load, like overstocked cichlid tank, discus tank, etc.

But any tank will be benefited the same way from this.

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