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Redundant Aquarium Filtration

Using More than One Filter on Your Fish Tank

(The First Tank Guide)

What Is Redundant Filtration?

The concept of redundant filtration is using two or more different filters on an aquarium, rather than just using one filter on its own.

Don't Multiple Filters "Compete" on the Fish Tank?

It is true that if you use more than one filter on your fish tank, neither of those filters will perform as efficiently as it would if it were the only filter on the aquarium. This is expected, but is not a problem.

Each filter is less efficient because of some of the restrictions placed on filtration systems by water volume and surface area. Both water volume and surface area will influence the density of waste and of oxygen in the water, and this will directly impact the efficiency of the biological filter over all. Because of the limitations on available oxygen in the water and the rate at which waste is produced and delivered to the filter, adding more filters will not increase the maximum population for your aquarium. This parallels the rationale behind not using bacteria boosters in an established fish tank.

Should I Use Redundant Filtration on My Fish Tank?

I almost always recommend using redundant filtration on your fish tank. The primary reason I recommend this is that if you have a filter fail for whatever reason, you have a backup in place that is already cycled and nearly capable of handling the load in your fish tank immediately. This can buy you some time to get to your local pet shop and get the parts you need to fix your other aquarium filter or to get a replacement for the failed filter, without leaving your tank unfiltered while you are doing this.

Why Not Use Multiple Filters on an Aquarium?

The most common reason I give for not recommending redundant filtration on a tank is if one or more of the alternate filtration methods is not appropriate for that tank or setup. That is not a recommendation against redundant filtration, but rather a concern related to one of the types of filters and the aquarium setup. Factors from location of the tank to the fish or animals you are keeping can impact whether one form of filtration or another is appropriate for your fish tank.

Will Adding Another Filter Increase My Maximum Tank Population?

The capacity of a biological filter, provided that you have sufficient filtration to support enough biological filtration for a tank the size of yours, is determined more by water volume and available oxygen than by the number or configuration of the filters being used. So, if you have a 50 gallon aquarium and a filter for a 30 gallon tank, your population will be limited by the undersized filter. However, if you have a 30 gallon aquarium with enough filtration for a 50 gallon aquarium, that fish tank really has the same maximum population as a 30 gallon aquarium with filtration appropriate to a 30 gallon aquarium.

An Example of Redundant Aquarium Filtration:

If you have a 120 gallon fish tank, and you have a power filter that is designed to support your 120 gallon tank, and you use only the power filter, you should have enough biological filtration to support your 120 gallon tank.

On the other hand, if you have a canister filter designed for your 120 gallon filter, and you use your canister filter, and you use a good biological filtration media in it so that it can provide biological filtration for your 120 gallon tank, you will also have enough biological filtration to support your 120 gallon tank.

If, on the other hand, you use both of these aquarium filters on the same 120 gallon fish tank, you will have enough biological filtration to support your 120 gallon tank - not enough to support a 240 gallon tank.

However, if you were to use these same two aquarium filters on a 180 gallon fish tank, you should be able to provide enough biological filtration for that 180 gallon aquarium.

The filtration capacity increases with multiple filters, but only if the water volume also increases.

Are There Cases Where the Filtration Will Not Scale?

However, as always, there are exceptions to this rule:

What this example is intended to demonstrate is that adding filtration to the tank, provided that the initial filtration was sufficient to the tank, does not increase the tank holding capacity in any significant way, nor does it use any one of the filter elements to its maximum capacity or efficiency. However, redundant filtration on your fish tank will be highly beneficial in any case where you need to disable one of the filters for a period due to a mechanical failure or due to medication.

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