Can You Use 2 Filters in a Fish Tank?

Any aquarium owner will have concerns regarding the cleanliness of the water in their fish tank. Owners see these come about for several reasons, such as the health of their fish, removing any waste accumulation from the water, and having a clear and unobstructed view of inside their tank.

We then have questions of whether a second filter can help with this, or will it cause more issues than it solves.

So, can you use two filters in a fish tank?  When you use two filters in a tank, this is known as redundant filtration.  Fish can benefit from doing this; however, it isn’t just a matter of adding a second filter to your tank and thinking all will be good. There are some considerations when you go about this, but on the upside, you will have a second filter to clean the water in your tank in the case of filter failure.

Read on, and there will be more relevant information about this subject. What you need to consider, what the benefits are, and what issues you may face by having a second filter in your aquarium.

What is Redundant Filtration?

When you look into this concept, you will have two different filters working in your aquarium. At the most basic, you have a spare pump which has already been cycled and will take over in the case of failure of the other pump.

In many cases, you may not notice anything different with your water, as the secondary filter is keeping the tank clean. Only when checking the filter is working will you see there is a problem.

In general use, it may appear to be overkill to use this redundancy filtration in place. However, if there is a failure, and there is no way to get a replacement or parts to repair, your fish and the rest of the tank will quickly suffer.

Why Shouldn’t I Use Two Filters in My Aquarium?

One of the significant concerns when adding a second filter is that of water flow. This does depend on the kind of fish you have, yet if there is too much flow, then some fish will not appreciate it, or take to swimming against a current all the time.

Betta fish are a prime example that prefers calmer waters, although there are some fish that like a gentle current. Tank size can play a large part in this, as a smaller aquarium will become more turbulent than a larger tank because of the water volume.

Before adding a second filter, you can do some homework as to what water conditions your fish prefer. Adding baffles is a simple and effective way of reducing flow, even with using two filters.

Beneficial Bacteria and Biological Filtration

It is vital to have some biological filtration in your aquarium. What this does is to help break down the ammonia content into nitrites, which are then broken down further into nitrites that are less toxic to your fish.

There have been debates on using two filters and over filtration as a result, and the effects it has on this bacteria. The beneficial bacteria grow in every area of your aquarium, yet the filter provides the ideal spot for it to grow. As water flows through this area, it makes contact with the bacteria.

The issue being, if your filter was too powerful, then you will lose the contact time of water to the bacteria. While you are looking at two filters, you will need to decide on the kind of filter to use and the placement. Two filters in one zone can create too much flow in one direction.

What are the Right Levels of Aquarium Filtration?

When it comes to deciding on the size of your filter, you do need to take into consideration the kind of filter you will be using. One of the most common filters in use for the majority of tank owners are outside filters.

These make use of a pump to suck water through a tube from inside the tank. Here, the water flows through a series of compartments and different types of media, before making its way back into the tank.

When using this kind, a general rule to follow is the filter will have a flow rate of four to six times the volume of your tank. A 30-gallon tank will have a 200 GPH (gallon per hour) filter as an example.

When looking at adding a second filter, you may think you can increase the size of your tank by double. Unfortunately, this isn’t the case, but you do need to be sure the total adds up to the size of your tank. It will be easier than making calculations because most filters will state the size of tank they can accommodate.

A second filter can be smaller; however, if you have a failure in your other pump, this redundancy will not be sufficient. If you have a 30-gallon tank as in the previous example, you can use two filters rated at 20 gallons without issues.

Can I Increase Fish Population with a Second Filter?

If you are looking to increase your fish population, then you will see that the number of filters doesn’t directly mean you can add more fish.

It is the capacity of the biological filter, which determines the number of fish you can accommodate in your tank. Factors that dictate this are water volume and available oxygen. Here, your fish population will be determined by your smallest filter.

As an example, say you have a tank which is 50-gallon, and you have a 30-gallon filter, you can have a population which the 30-gallon filter can cope with. Likewise, if you have a filter rated at 50 GPH and your tank is 30-gallon, you can still only have a population that is comfortable up to the 30-gallon maximum.

You will find that your filtration capacity will only increase as the water volume increases, so you will need a tank that can hold up to its maximum rather than trying to squeeze more fish inside because you have a larger filter.

Filter Location

We already see we shouldn’t have two filters in the same vicinity. It will only increase the current and flow, so it is wiser to have them in other areas.

Not all water from inside a tank is going to make its way through the filter. The way they work, and the way water works are, returned water will be sucked back into the inlet tube.

Across the tank, you can find dead spots. These areas have little chance of being filtered, and the water can become stale. It is these areas that create organisms that are harmful to fish.

While, in theory, a filter can pull all your water through the pump several times per hour, you need to think about any plants, rocks, or tank decorations you have that block the flow of water toward the filter.

You can now see where a secondary filter shows its real benefit.

Should I Get the Same Kind of Filters?

If you are choosing a secondary filter, there is little sense in purchasing one, which is the same type.

There are three types of filters you can choose, so it is advisable to use two that offer different benefits. As some are multi-purpose, you can cover all three types in two when you shop around.

  • Mechanical.  This filter eliminates dirt out of the water.  The mechanical filter captures wastes from fish, unconsumed food, and other residues in the water.  Maintaining water cleanliness in a tank is critical to building a healthy ecosystem, so mechanical filtering is essential.
  • Biological.  Ammonia and nitrites discharged from fish have to be broken down by beneficial bacteria.  For this to occur, you need rocks or sand, so the good bacteria will have something to cling to.  The bacteria also require oxygen-rich water.
  • Chemical.   The use of a medium like activated carbon to dispose of waste is useful.

Power filters are ideal when used in conjunction with something like an internal canister filter such as the MarineLand. These require no priming and are easy to get additional cartridges for the triple filtration system.


When you weigh up the pros and cons of a second filter in your fish tank, the pros outweigh the negative points. All you need to remember is that too much flow can be uncomfortable for your fish.

Add to this, having two filters that are both rated for the capacity of your tank, will be a waste of energy. You will then have two running in the eventuality of one failing, and the second taking over as a backup.

Be sure your flow rates are slightly above your tank maximums, and then you will have a host of happy fish in your aquarium.