How to choose the best filter for your aquarium

The cleanliness of aquarium water relies on a functioning water-filtration system. But there are so many of them, that it is easy to get confused. In this article, you will learn the basic concepts of maintaining clean water in your fish tank with the help of aquarium filters. We will explain the stages of filtration, and familiarize you with the most common aquarium filter types.

Aquarium filter pipe and a fish
Photo: Maria Itina

What is filtration? Most people understand it to be the removal of waste. But filtering is actually more than just that, and consists of three steps.

  • mechanical filtration removes solid and dissolved particles from the water.
  • biological filtration converts fish waste products into plant nutrients with the help of microorganisms.
  • chemical filtration removes other wastes and contaminants that could not be removed in the previous steps, most commonly with a help of activated carbon,

Most aquarium filter types provide all three of those steps. But which type is the best choice for your particular needs?

Most common aquarium filter types

Below is a list of most common aquarium filtration systems. If you are just starting with an aquarium hobby, you better go with one of these.

  • Internal submersible filters are suitable for aquariums up to 50 gallons (200 liters) and are the easiest to use. If you have a small fish tank, we recommend you to go for this option. The filter is an easy-to-install, standalone device, which is attached to the inside wall of an aquarium and pushes water though its sponge-like filter media. First, it is purely a mechanical filtration. However, once bacteria start to grow on the media, it also provides a biological filtration. Most filters have two to three media pads that can be taken out, rinsed, or replaced when needed, and some filters have an option to put in a media that contains activated carbon, also providing a chemical filtration.
  • External canister filters sit below the fish tank. They take in and then return water to the aquarium through attached hoses. Obviously, such filters require a harder setup, and they usually cost more that internal filters. The added benefits, however, are that they can be cleaned without putting your hands in the fish tank and that their filter media can be customized. They usually contain several slots for filter media, which unlike internal filters, can provide several mechanical filtration stages (coarse, medium and fine)—one after the other. This fact makes it more efficient and prevents clogging. You can also add a filter media special for your needs. For example, some fish species require slightly acidic (pH < 7) water. In such case you can insert a media containing peat, which acidifies the water on the go. The same holds true for other parameters that cannot be bought out of the box. If you test and find that your water is rich in phosphorus (a common cause of extensive algae growth), add media to reduce that.
  • Clip-on power filters are similar to external ones, only they are clipped on the side of tank and sit slightly above it. There are no hoses to attach, but not all fish tanks have a space for them if you already have a hood on top of the tank. It is a common choice for out-of-the-box starter kits for beginners, since the hood and everything else is already made for the specific filter. Note, though, if it breaks at some point, you will need to replace it with an exact same filter model or replace the fish tank’s hood.
  • Gravel filters use the gravel as a filtration media. These filters must be installed during the setup of an aquarium, because a large plate is installed beneath the gravel. A large surface area means improved biological filtration. Biological material trapped in the gravel also means more nutrients for the plants, and there is also an added benefit that the filter is not visible; there are tubes that run along a back wall and corner, but they can produce bubbles, making them actually cool. It may be a struggle to set up a gravel filter, but, when done right, it works like a charm.

There are other filtration systems available, though not as common. Most beginners are likely to deal with internal or clip-on filters. If you come across a system not mentioned here, do not hesitate to ask the salesperson to explain how it works and seek information on the manufacturer’s website.

Note that whatever filtration system you choose, it relies heavily on beneficial bacteria that provide biological breakdown of waste-products. This means, that, whenever you set up an aquarium, you should run it for several days before adding fish so that the bacteria have time to multiply and colonize the filter media. More on this when we talk about setting up an aquarium.