How Often Should I Change My Aquarium Filter?

How Often Should I Change my Aquarium Filter

A properly working filter is vital for the health of the fish living inside the tank. Filters keep water clean and sparkling and also remove particles floating in the water.

 Extra food, chemicals, medications, decaying organic matter, and waste products can all be removed by a filter. There are three categories of filters: mechanical, chemical, and biological.  

Filters should be cleaned and maintained on a regular basis but each type of filter will have different requirements. Mechanical and biological filters will only need to be replaced when they stop working or fall apart. 

Chemical filters will usually need to be changed once a month. 

What Does a Filter Do?

Filters are an essential part of a fish tank, helping to keep the water environment safe and healthy. If waste from fish droppings is allowed to accumulate, levels of ammonia can build up, poisoning the fish.  

Uneaten food and decaying organic matter from plants or fish scales can cause water to appear cloudy and murky. Toxins and other chemicals like medications or additives in dechlorinators will also be cleared by filters. 

filter for aquarium

Aside from taking harmful products out of the water, filters are beneficial by placing something in- bacteria. Colonies of bacteria build up on the media of filters, and they are responsible for converting the ammonia in fish waste to nitrates and then into nitrates. 

This end product is fairly harmless to fish and can be removed with water changes. Therefore, it is important not to clean a filter too often or too completely in order to spare the bacteria that has made the filter their home. 

Types of Filters

Filters are divided into three categories depending on how they process water. Each type of filter has different cleaning and replacement needs. Mechanical filters physically push water through some type of media, straining the particulate matter out. 

Biological filters use bacteria or plants to convert harmful products into beneficial ones. Chemical filters use a substance like carbon to bind toxins in the water. 

Within each of these three categories of filter is found different types, both submersible and non-submersible. Canister filters draw water into an enclosed canister and pass it through a variety of filter media. Under gravel filters pull water down and through the gravel at the bottom of the tank. 

Sponge filters use air to power them and pull water through a sponge. The type and number of filters used in an individual tank will vary by size and species of fish. However, it is recommended to have a variety of filters for back-up in case one fails. 

dirty aquarium

1. Mechanical Filters

Mechanical filters push water through some type of media which strains the water and frees it of particulate matter. Types of media may be sponge, filter pads or floss, or even gravel.  

How often these will need to be cleaned and replaced depends on the volume of water, the rate of flow, and how small the holes in the media are. Fine filters will clear more water of particulates but will clog easier. 

As the filter becomes dirtier, its efficiency will decrease. This will be apparent by cloudy water with visible debris. 

It is recommended to clean mechanical filters by rinsing once a month. Cleaning too often will reduce those good bacteria colonies. Often filter cleaning is combined with water changes; using the tank water to rinse the filter will keep those bacterial colonies intact. 

Mechanical filters will only need to be completely changed if the media which strains the water is wearing out, falling apart, or missing. It is a good idea to put the new filter in the tank for a few days before removing the old filter in order to give the new media a chance to accumulate bacteria. 

2. Chemical Filters

Activated carbon is responsible for removing impurities in these types of filters. In general, 1 cubic inch of activated carbon can clean 2 gallons of water for a time period of up to 1 month. 

After this time the carbon media will become full of particles and will no longer be able to keep the water clean.

Also, the type of water in the tank can affect the life of the carbon filter. Hard, treated water will cause the filter to become saturated faster. Carbon filters that have become full can be very harmful. 

They may even begin to release any toxins or chemicals that are not bound tightly to the media. Therefore, it is critical that these types of filters be replaced every month. 

3. Biological Filters

Bacteria, plants, and fungi will filter any water that naturally passes over them. Bacteria will normally grow on surfaces, including filters, and are beneficial for maintaining the nitrogen cycle in the water.

 Ammonia from fish waste is broken down by bacteria into nitrites. These nitrites are then converted into nitrates which are not as harmful. 

Plants are able to use some of these nitrogen compounds as food during photosynthesis, but the majority of nitrates will be removed during water changes.  

Although bacteria will grow all over the tank, including in the gravel, specific biological filters can be purchased. These provide a separate area for bacterial colonies to attach to. These filters will need to be replaced once they start to deteriorate.   


Filters are critical for extending the life of fish by providing them with clean and pure water to live in. It is important not to clean or replace filters too often in order to preserve the helpful bacteria colonies growing on them. 

There are three categories of filters and each has different cleaning and replacement requirements. Mechanical filters pass water through a strainer. They should be cleaned once a month and replaced when worn out.

 Chemical filters use carbon to bind toxic chemicals in the water. They will need to be replaced every one to two months because they will become full and start to leech carbon and toxins into the water. 

Biological filters use bacteria, and to a lesser extent plants, to remove harmful nitrogen compounds from the water. Replace biological filters when they become damaged or fall apart.

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