Aquarium Filtration Basics

A good filter is a must-have for any aquarium. It helps keep your fish healthy and your water clean. But understanding how aquarium filtration works can be a little confusing. Filters come in many different shapes and sizes, from air pumps to wheels. And they use different methods to keep your aquarium water clean. The right filter for your fish will depend on your aquarium size, feeding and maintenance habits, and the type of fish you have.

Why Your Aquarium Needs a Filter

Keeping your aquarium water healthy is a delicate balancing act. Because your fish release their waste in the same environment they swim around in, filtration is vital for their well-being. Waste breaks down into components like ammonia that are toxic to your fish. Suspended particles can make the aquarium unsightly. Dissolved waste can throw off the water's chemical balance or even lead to an overgrowth of algae. Persistent filtration is necessary to keep everything running smoothly.

Types of Filtration

Aquarium filtration has three main components or stages: mechanical, chemical, and biological. Some filters may utilize only one of these, while others may utilize two or even all three.

  • Mechanical filtration uses floss or foam materials to trap larger particles suspended in the water.

  • Chemical filtration binds dissolved particles to a particular media to remove them from your aquarium water. It often uses activated carbon but might also use resins or clays.

  • Biological filtration (nitrification) converts toxic ammonia from fish waste to nitrite (which is also toxic) and then converts the nitrite to nitrate. Nitrate is safer unless it builds up to very high levels.

Some filters may also use ultraviolet (UV) or ozone systems to aid with cleaning the water.

Different Types of Filter Products

Many types of filters are available, and some aquarium owners combine different types to maximize filtration. Below is a look at some of the most common.

  • Canister filter: This uses a water pump to force water through different types of substances that create mechanical, biological, and chemical filtration. Needs frequent cleaning but is great for larger aquariums.

  • Fluidized bed or sand filter: Tank water is pumped up into a sandbed. The rolling water keeps the filter media (silica sand but sometimes resin beads) constantly moving (biological and mechanical filtration). Works efficiently.

  • Power filter: This pulls water out of the tank, forcing it through a cartridge with various media that filter the water. It often has floss that traps solids (mechanical filtration) and activated carbon (chemical filtration).

  • Nitrifying bacteria: Nitrifying bacteria can also grow (biological filtration). You must replace or clean the cartridge regularly.

  • Rotating biological contactor or bio-wheel: A wheel made of porous mesh rotates, dipping in and out of the tank water and the air. It provides oxygen for nitrifying bacteria growth.

  • Sponge filter: An air pump circulates water through a foam sponge (mechanical filtration), and bacteria grows on the sponge (biological filtration). Best for small tanks with fewer fish.

  • Under-gravel filter: This uses an air pump to pull water through a slotted plate under your aquarium's substrate, then circulates the water to the top of your tank. Mechanical filtration traps solids in the substrate, and biological filtration lets nitrifying bacteria grow on the substrate. Best for tanks not densely packed with fish.

  • Wet-dry filter or trickle filter: This drips aquarium water over a medium for maximized oxygen saturation and biological filtration. A wet-dry filter is great for large tanks.

Filters are often combined for greater filtration and efficiency. For example, you might use a sponge filter alongside an external power filter.

Components of a Filter

The components of your filter will depend on the type of filtration you need. But it will always include the media you're using to filter out particles, such as activated carbon or sand. Some filters may even include empty modules so you can choose extra media for your particular needs, like resin (phosphorous removal), zeolite (ammonia removal), peat moss (to acidify water), or crushed coral (to raise the pH of the water). Or maybe you'll add a bio-ball for enhanced biological filtration.

Your filter might also have modules in graduating pore sizes, so you can trap large particles first and increasingly fine particles as water moves up the filter.

You need to keep some replacement parts on hand too, such as extra filter cartridges (your filters will eventually get dirty and need replacing). For example, if you have an Aqueon QuietFlow filter, you need specialty filter pads. You can choose between a carbon pad, an ammonia reducer pad, or a phosphorous reducer pad.

You may also need a vacuum or siphon to help clean your aquarium's substrate, especially if you're using an under-gravel filter.

Maintenance of Filters

Regular maintenance is vital to keep your filters working properly and your fish healthy. But the maintenance requirements can vary a bit depending on what type of filter you use.

  • Mechanical filtration filters need to be cleaned regularly to remove organic debris that breaks down and releases ammonia.

  • With chemical filtration, you may need to replace the absorbing media every four to six weeks, depending on the size of the filter, your water, and how many fish you have.

  • If you're using an under-gravel filter for biological filtration, you need to vacuum the substrate in your aquarium frequently.

If you don't clean your filter regularly, you might end up with an overabundance of algae in your aquarium. Regular cleaning can help lower high nutrient levels and maintain optimal water quality. However, it's important to remember that regular filter cleaning is not a substitute for partial water changes, which are also important.

Product Recommendations

Choosing the right filter for your aquarium isn't always easy. Smaller tanks might simply need a small filter that uses pumped air to circulate the water, like a sponge filter. If you keep larger fish that produce a lot of waste, then you might need a stronger external filter, like a power filter.

You might even want to combine more than one filter for greater filtration and efficiency. For example, you might use an under-gravel filter alongside an external filter like a power filter. Some aquarium owners set up mechanical filtration in front of a different type of filter so the larger particles are filtered out first and don't clog the second filter.

Examples of great filters to consider include:

Choosing a filter isn't easy. But knowing how filtration works and why filters are so important can help you better understand which type of filter is best for your aquarium.