Choosing An Aquarium Filter

Crystal clear water that looks like it has been polished can be achieved with proper filtration. Getting it wrong, on the other hand, can lead to cloudy, dirty, smelly water that will only do harm to your goldfish.

With so many different types of filter available – each with very persuasive marketing all over the packaging – it is hard to know which is the right one for any given goldfish tank.

How Filtration Works

Mechanical filtration – this is simply the process of making water pass through a substance that will stop particles of dirt, faeces, uneaten food and other solid matter that are carried in the water. Examples of mechanical filtration include mesh, netting, sponge and filter wool.

Biological filtration – this is the process of turning poisonous ammonia (a by-product of decaying plants, food and goldfish faeces) into less harmful nitrates. These nitrates are then removed by regular water changes. Biological filtration is performed by bacteria that colonize the large surface area provided by sponges, gravel and filter wool. These bacteria can also be present in specific plant roots. Sometimes the mechanical filter and the biological filter are contained in the same physical item – e.g. a sponge – and sometimes they are two separate entities i.e. the water passes through a mechanical filter, then a biological one. Biological filters should only be cleaned in tank water, never in running tap water.

Chemical filtration – this form of filtration is used to remove chemicals from the water that are dissolved rather than suspended in the water, and that cannot be broken down by bacteria normally found in goldfish water. The most popular form of chemical filtration is activated carbon, that removes a number of toxic chemicals from the water, and is often used after adding medicinal chemicals to water. Another example is ammonia remover. Although biological filtration usually deals with ammonia, sudden increases in ammonia can overwhelm a biological filter.

Types of Filter

There are several different types of filter available, each with its own advantages and disadvantages. The following list summarizes the most common types.

Box filter – a very cheap, small plastic box that sits in the back corner of an aquarium, and is filled with filter wool and/or activated carbon. This type of filter is only suitable for very small tanks with very few goldfish in them, or in spawning tanks. It can also be used in larger tanks for quick and easy chemical filtration if it is filled with activated carbon.

Under gravel filter – a tray the same size as the bottom of the goldfish tank with one or two upright tubes attached to the back corners. It needs to be covered with gravel, because it is the gravel that acts as a mechanical filter, and provides the surface area for the ammonia-nitrite-nitrate bacteria to colonize. An under gravel filter is also cheap, but it is not very efficient and causes problems for aquarium plants.

Internal power filter – a two part filter consisting of a pump and a chamber containing a sponge. The sponge acts as both mechanical and biological filter. It provides good circulation and oxygenation of the water, and the sponge makes a very happy home for biological filter bacteria. However, the surface area is relatively small, so it can’t be used in large tanks. Also the filter itself may be a bit unsightly in the tank.

External power filter – works the same way as an internal power filter but sits outside the goldfish tank, often hanging from the top edge of the back of the tank. It is a popular choice for small aquarium owners as it gives the inside of the tank a more natural look, but it has the same limitations as an internal power filter.

Canister filter – this is a large external canister that works well in large aquariums. Most canister filters come with mechanical, biological and chemical filtration as standard. They work extremely well and hold a lot of filter medium, so don’t need cleaned very often. They are however for the more serious goldfish enthusiast, and this is reflected in the price. Also, they are not small and need room. This will often mean a custom-built cabinet to hide the canister.

Diatom filter – used for one off cleaning of the water, a diatom filter is a mechanical filter with a microscopically fine filter medium that can filter out almost all physical impurities in the water, right down to micro-organisms. A diatom filter cannot be used continuously, the medium clogs up too quickly. It is used at regular intervals to keep the water crystal clear.

Wet/dry filter – more likely to be found in large saltwater aquariums than goldfish aquariums, wet/dry filters work like sewage treatment beds. Water is trickled or sprayed onto a mechanical filter medium which provides high levels of oxygenation. It then passes on to the biological filter, and possibly a chemical filter. This type of filter is the most expensive and requires the installation of plumbing.


For most goldfish owners an internal power filter or an external power filter will do very well, with a box filter on hand for chemical filtering when needed. However, it is important to choose filters that will put enough water through to actually keep the water clean. At least 4 times the volume of the aquarium per hour, and more if the tank is heavily stocked.

Also, test the aquarium water regularly to make sure ammonia and nitrite levels are not rising. If they are, it could be a sign that biological filtering is not coping. If the water is cloudy or dirty, then the mechanical filter needs upgraded.

4 Responses to “Choosing An Aquarium Filter”

  1. water filter Says:

    also, whenever you overfeed your fish, like when you put so much food on the aquarium, the food goes to the bottom and makes the water dirty. some of them takes a long time before they get dissolve and your water filter really doesn’t clean the water well.

  2. BIG BALLS Says:

    the ONLY aquarium filter i will use is the EMPEROR 400 i have 11 aquariums and its all i use on each the larger tanks have 2 running. i have never had such crystal clear water and they keep the tanks perfect 24/7. i buy the filters on EBAY and i encourage everyone to try them. they have 2 bio wheels and filter 400 gallons a hour. i use 1 filter on my 30 gallon tanks and 2 on my 75 gallon tanks. my goldfish are all healthy in all my tanks and growing like crazy. i have ranchu’s, oranda’s, bubble eyes, moor’s, fantails, just about every type in the various 11 aquariums.

  3. FilterGuy Says:

    Nothing beats canister filters. The Emperor 400 is a decent filter. However, even the cheapest canister filters beats the 400 in terms of the sheer filtering power.

  4. sanjay Says:

    I have long admired planted aquariums. But, I have never been unable to find any good strategies on how to set one up!

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