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Aquarium filters - header image

Differences between internal and external aquarium filters and diagrams, FAQ and Forum

Brief Description

This page is an ultimate guide that answers questions regarding internal and external aquarium filters. However if you'd like to purchase fish tank filters online, simply follow this link. This article contains FAQ with answers, pictures and forum with personal experiences of aquarists from all over the world. We'd love to hear what filter you're using, so submit your story at the bottom of this page, please!

Both internal and external filters are good solutions to keep clean water and good chemical values, there is not a better one, it depends on different factors connected to the aquarium.

Internal filters

Internal filters usually are rectangular boxes attached with suckers on a side glass of the tank. They may have two or three sections inside, thought to divide filter materials, pump and sometimes also the water heater.

Often you can find in shop really small internal filters meant for 10 gallons (45.46 litre, 12.01 US gallon) aquariums, even if they claim to use special technologies, they are the most of times inadequate for the aquarium cleaning.

Adding plants and reducing the number of fish inside the tank is not a solution with inadequate filters, specially in small aquariums; an undersized filter can not guarantee constant chemical values, even if it can clean water, great sudden changes of nitrite, nitrate, and ammonia due to the small concentration of filtering bacteria, can kill your fish and plants.

A good rule to follow to understand if an internal filter is of the right size for your tank is to choose a filter almost as big as the smaller lateral glass of the aquarium.

Filtering materials are a personal choice anyway is always better to use two different filtrations, a mechanical one and a biological one.

In case the filter is divided into three sections, put in the way in section some foam to block the passage of leafs, food rests, and other big refuses. Fill the middle one with ceramic rings or other biological filtrations, in this part will live the most of the filter bacteria. Add some other foam in the third part before putting the pump, to avoid big refuses left to come inside the pump.

The cleaning of internal filters can be difficult. Some of them have the sections formed by baskets easy to take off, separated without dirtying the water. In case your filter does not have the possibility to be cleaned in separated parts, change every 20 days the foam, in case it still looks good you can also clean under water and then reuse it again. The biological filter must not to be changed often, because the most it stays there the higher is the number of filtering bacteria inside; in case you see it really dirty you can take off and change just a part of it.

Internal aquarium filter scheme

External filters

There are a lot of external filters, the most used are two:

  • classical external filter
  • hang on external filter

The classical external filter is formed by a hermetically closed container connected to the aquarium by a way in and a way out pipe. They can be placed near the tank or under it, and usually are employed for bigger aquariums.

The hang on external filter is a quite new filtering system. It consists of a rectangular box that hangs on the aquarium glass by a curved side. The pump is situated in the way in and gets water with a solid pipe, while the way out water creates small falls passing on the curved hang on side of the filter. These filters are really nice to see and are thought for small aquariums, anyway due the hang on part they need to be placed on open aquariums, or you have to modify the closure of the aquarium top.

External filters usually provide a good water cleaning and chemical values control. Each brand has a different design of the internal structure of the filter, anyway usually both classical and hang on filters are divided into small removable baskets or sections. Surely external filters are easier to clean because during this operation is impossible to dirty the aquarium. Moreover often classical external filters have the possibility to close and leave attached at the aquarium the way in and way out pipes, so you can clean the filter in another room, such garden or bathroom, without pouring out dirty water around.

Some external filters have the pump positioned on the way in, the problem of these models is that big refuses can damage the engine of the pump. Usually they come with a grid to block these refuses, make sure that the grid works fine, in case of doubt you can put a small part of foam before the way in.

External aquarium filter scheme Hang on external aquarium filter scheme

How to choose the right filter for your aquarium

As mentioned above, there is not an universal best filtering system, it depends on the characteristics of your aquarium.

  • If you are buying your first aquarium, you surely find easily tanks with internal filters, in case they are big enough, as explained before, you can use them without problem.
  • In case you have a small aquarium with internal filter and you are experiencing problems related water pollution, be sure to have a working biological filter and that your aquarium is not overpopulated, before changing the internal filter with an external one.
  • External filters have usually most powerful pumps, so in case of small tanks, prefer a hang on one, because they have been designed for small amounts of water.
  • The use of oversized filters, specially external, can surely solve pollution problems, but the water current is more powerful and can make fish sick and be a cause of algae development.
  • If you do not feel to clean the filter often and prefer a easier cleaning solution, prefer a classical external filter.
  • If there are small kids or pets that could trip against aquarium accessories, prefer an internal filter.
  • Big aquariums with low water level, like paludarium, or tanks dedicated to frogs or turtles prefer classical external filters that work fine even with a low water level.

Copyright note: This article is originally written by Michela Ferretti. owns the full copyright of this article.


We are not advertising any product. Provided images are informational only. Aquarium filter Aquarium filters, picture 2

Fluval 305 external aquarium filter, picture 1 Fluval 305 external aquarium filter, picture 2 Fluval 305 external aquarium filter with cartridges

Additional information, questions and answers

As you can see below the article, we keep answering questions that visitors of Aqua-Fish.Net ask. Due to merging our section with related articles we've updated this page on March 18th 2011 once again and here below you can find usual questions along with answers. Some of the questions may have been answered above, or right below, while some may be answered only partially, and most of them weren't answered on this page up until March 18th. If you still cannot find answer on your question here, feel free to use the form that can be found at the bottom of this page!

  • Which media should I use in my external filter?

    Answer: Your external should hold media that will cover all aspects of filtration-mechanical, biological, and chemical. Mechanical filtration needs sponges adding to catch the debris, biological filtration is performed by adding ceramic rings or bio balls, chemical filtration is performed by adding carbon.

  • What size sponge filter for 10 gallon (45.46 litre, 12.01 US gallon) tank?

    Answer: The pedestal sponge filters vary in size but will have their rating guide on the packaging. If you are using the basic sponge filters that clip on the side of the tank I always suggest getting the double sponge filter as it gives the option of cleaning each sponge at separate times.

  • Where do I buy a sponge filter for a breeding tank?

    Answer: Local pet stores often carry sponge filters, and they can be found on many on-line stores such as

  • How do I know if I need an external filter?

    Answer: Internal filters can cope with the water volumes held in smaller aquariums. When the water volumes get above a certain level you will need to invest in an external filter. All filter suppliers will provide a table stating which model of filter is best for which water volume.

  • What do carbon filters do for the aquarium?

    Answer: Carbon filter inserts, removes and neutralizes dangerous chemicals such as ammonia, or even fish medication.

  • Why do a lot of bubbles come out of my aquarium filter?

    Answer: The filter must be sucking in air from the atmosphere; this is mostly caused by the top of the filter being too close to the water surface.

  • Where are filters placed in an aquarium?

    Answer: There are a few different kinds of filters: (HOT) hang on tank, canister, internal magnetically driven, and internal air driven. Hang on tank filters are generally placed on the back side of the tank and are probably the most common type. These have a tube that lowers over the side of the tank into the water and once primed (filled with water) will "suck" water through physical/chemical media purifying the water. Canister filters are similar to HOT filters in that they have a tube that extends up and over the side of the tank siphoning out water and passing it through filter media and returning it, via another tube, to the aquarium. These are easy to spot as they have a canister that sits behind or under the aquarium, and a siphon hose and return, it is also important to note that the higher end ones also have a shutoff valve that makes cleaning much easier. Internal magnetically driven work similar to the previous two types, except it is placed into the water, submerged to the manufactures recommendations it pumps the water through filter media within the tank. This filter will have nothing outside the tank except the power cord. Air driven filters work on a displacement principal. The air creates a suction action as it passes through the tube towards the surface. This drags water through the filter media or aquarium substrate purifying the water.

  • Which external filter should I use?

    Answer: The type of external filter you use should depend on the size of aquarium you have. For the smaller size aquariums it is perfectly OK to use a hang on back filter. They are relatively inexpensive, they are easy to maintain, and they are designed to move smaller quantities of water. For larger aquariums with more fish there are canister filters. They can be more expensive but they are designed to handle larger volumes of water with more waste loads.

  • Which fish can live without filter and oxygen?

    Answer: Labyrinth fish such as bettas and gouramis are able to survive without a water oxygenating source. These fish have a labyrinth organ that gives them the ability to breathe atmospheric oxygen. This allows them to live in murky, oxygen depleted bodies of water where other fish would never survive.

  • Why do you get high nitrite levels in the aquarium?

    Answer: The first reason for high nitrites is when a tank is not fully cycled and the filters have not built up large enough colonies of bacteria to convert the nitrites to nitrates. The second reason is usually when the filtration is not capable of coping with the stocking level of fish in the aquarium.

  • Why does my tank filter stop when we unplug it?

    Answer: Unless you are using air driven sponge filters, all tank filters require electric to run, cutting off the supply will stop the filters.

  • Which filter is the best for aquarium fish?

    Answer: The best filter for aquarium fish depends upon who you are asking. Opinion has a lot to do with how great a filter is. Since there are many different types of aquariums, each with a different set up and different needs, the best aquarium filter depends on your own personal preference.

  • When should I change the filter media?

    Answer: The filter media should be changed at intervals recommended by the manufacturer. Try to stagger changing the different media so that it is not all done at the same time. Changing it all at once will reduce the efficiency of the filter.

  • Which external aquarium filter is best?

    Answer: The best external aquarium filter is the Rena FilStar xP. It is extremely silent and powerful, and it comes in a variety of sizes. It is a canister filter that sits beneath the aquarium, concealed in the stand. This filter can be costly compared to some of the other options but it is well worth it.

  • Which filter media works best?

    Answer: A good quality activated carbon would be the best filter media. Use one that will effectively filter chemicals and other substances from the water. The better the quality of the carbon, the more effective its filtration ability will be.

  • Which aquarium filtration system should I use?

    Answer: Use a filter that is meant for use in fish tanks that are about double the size of your aquarium. By doubling the water filtration, you are increasing the amount of filtration media while doubling the amount of water displacement. This will ensure better maintenance of the aquarium.

  • Where should the water level be on an aquarium filter?

    Answer: Water level on hang on tank filters should be at the top or just over the top of the filter media. If the water is well over the media, it may be time to change or clean the media. Air driven sponge or under gravel filters should be completely submerged to cover all the media.

  • The water flow from my filter is too strong, how can I turn it down?

    Answer: On all modern electrical filters there should be a lever or slide that can be moved to increase or decrease the amount of water flow coming out of it. If you are using air driven sponge filters, add a flow restrictor to the airline, this will control the filter.

  • Which filter is more effective: hang on back or internal filter?

    Answer: Internal filters tend to be less effective when trying to effectively move large volumes of water. More internal filters would need to be placed through out a large aquarium in order to filter the same volume of water as a hang on back (HOB) filter. HOB filters are more efficient and take up a lot less space in the aquarium.

  • Why shouldn’t you clean your filter sponges with tap water?

    Answer: Cleaning the sponges with tap water will kill off the colony of beneficial bacteria need to filter the water. Always use old tank water to rinse out the sponges.

  • Can I turn off the filters for 2 hours when I am cleaning the tank?

    Answer: A lot of keepers do turn off their filters during a tank clean including myself. Turning them off for 2 hours will do no harm but do not remove them from the water.

  • When should I change the aquarium filter sponge?

    Answer: Filter sponges come supplied with manufacturers instructions as to when to replace the filter sponges. This is mostly after they have been in use for 6 weeks but read the guidelines before use. Regular cleaning of the sponge with old tank water will still need to be done on a regular basis.

  • Which aquarium filter is good?

    Answer: The size of the aquarium will help to determine the best type of aquarium filter to use. Smaller aquariums that are 10 gallons or less can be maintained with either an under-gravel filter or an air powered filter. These filters are only useful in situations where the fish are not very messy and there is not much debris. 30 - 55 gallon (136.38 - 250.04 litre, 36.03 - 66.05 US gallon) aquariums will do better with a hang on back or internal power-head filter. The moderate water displacements for these systems are perfect for these aquarium sizes. Any aquarium larger than 55 gallons (250.04 litre, 66.05 US gallon) would benefit greatly from having either a canister filter or a sump. Both of these systems can be set up to filter larger quantities of water in shorter periods of time because they are generally larger than the other filtration systems.

  • When do I need to clean out my canister filter?

    Answer: Cleaning of canister filters should be done on a regular timetable of every couple of months. If the water flow reduces significantly in between the schedule, clean out the canister as it has become blocked. Always use old tank water for cleaning purposes.

  • Where does a filter go on an aquarium?

    Answer: There are a few different kinds of filters and each would be placed in a different location. Some examples are: (HOT) hang on tank, canister, internal magnetically driven, air driven (internal). Hang on tank filters are generally placed on the back side of the tank and are arguably the most common type. These have a tube that lowers over the side of the tank into the water and once primed (filled with water) will "suck" water through physical/chemical media purifying the water. Canister filters are similar to HOT filters in that they have a tube that extends up and over the side of the tank siphoning out water and passing it through filter media and returning it, via another tube, to the aquarium. These are easy to spot as they have a canister like part that sits behind or under the aquarium, and as noted, a siphon hose and return- it is also important to note that the higher end ones have a shutoff valve that makes cleaning easier. Internal magnetically driven work similar to the previous two types, except it is placed into the water, submerged to the manufacturers recommendations, it pumps the water through filter media within the tank and this filter has nothing outside the tank except the power cord. Air driven filters work on a displacement principal. The air creates a suction action as it passes through the tube towards the surface. This drags water through the filter media or aquarium substrate purifying the water.

  • Why are canister filters so good?

    Answer: All types of filtration are good if used to the correct rating but canister filters are better for large aquariums as they can hold a lot more filter media and filter more water per hour. Always check the rating before purchasing your canister filter.

  • What is a canister filter?

    Answer: A canister filter is a 2 or 3 step filter that is outside the tank, instead of hanging on the tank or completely inside the tank like under gravel filters.

  • What causes a noisy fish tank filter?

    Answer: Usually some impurity. These cause a filter to become dirty from inside, thus it is having problems filtering the water. Impurities don't help the propeller to work properly. Clean the filter and it should work quietly. If it's still noisy, there may be some serious failure of the engine and it is recommended to use another filter instead.

  • What size of power filter do I need for fish tank?

    Answer: The size of the filter required depends on the water volume of your tank. With all filters there will be a rating system for the size required. Normally internal filters will be fine with tanks 3 feet or less. Over 3 feet larger canister filters will be required.

  • What type of filter is best to use with fish fry?

    Answer: To keep the fry from being sucked into the filter, a fine pore sponge or cheese cloth should be fitted over your filter intake. Alternatively, you may remove the fry into a separate aquarium that has only an air driven sponge filter or well cycled under gravel filter. Take care not to have the fry out of water for long, and if possible not at all. Chasing or scooping them up within their aquarium is best.

  • What fish can I keep without a filter?

    Answer: This depends on the size of your aquarium. Imagine a 200L aquarium with 2 Siamese fighting fish in it and say there are plants too; Then you don't need any filter because plants will decompose all excrements. But if there are more excrements than plants can decompose at the time, then you surely need a filter. Do not forget that even filter is not needed in all aquariums, water current must exist. Without proper water current the pH level can become very unpredictable very fast (usually lower than 6) - however, pH is also closely related to carbonate hardness.

  • What is meant by the term "media" in a fish tank filter?

    Answer: Media is simply the physical or chemical component of your filter. Physical filtration is usually a screen, sponge, cheese cloth, or floss intended on catching larger waste, and suspended particles. Charcoal/carbon is a chemical filter and is usually made from baked coconut shell (or other wood like material) and appears as black granules. It works by trapping chemical or physical material in microscopic cracks caused by super heating in the manufacturing process. It also works as a biological filter by providing surface area for beneficial bacteria to grow on. Another common pouch found is an amonia-carb pouch (white or white and black granules) which is simply a pouch filled with an aquarium safe chemical which would bond with the ammonia produced by the fish waste. It would act as a biological filter the same was as carbon/charcoal. Finally, some other types of media sold on the market are bio rings and bio balls; both would act as part of your biological filter. Peat pouches or peat moss would be used to change the pH of the water and to create/convert water to be "black water". If you would like a simplified answer: anything intended to be used as a insert into a filter could be called filter media.

  • Why do some aquariums need canister filters?

    Answer: All aquariums need to have the water filtrated for the highest of water quality. In larger tanks internal filters are just too small to cope with the bio loads. Canister filters have larger surface areas for the filter media so should be used with the larger water volumes.

  • Where can I buy sponges to make sponge filters?

    Answer: Just about any sponge can be used to make a sponge filter. just be sure that they are not impregnated with any antibacterial or antifungal agents as they will be harmful to your fish. Usually this means the cheaper the sponge the better. Plain "jumbo sized" yellow or white sponges are often found in department stores and can be cut up many different ways and shapes. It is important to note that most aquarium sponge filters are engineered to catch specific size particulates and achieving the same performance with DIY sponges may not be possible.

  • What size filter do I need for a 16 gallon (72.74 litre, 19.22 US gallon) tank?

    Answer: I normally use filters that are rated for twice the aquarium water volume per hour. For a 16 gallon (72.74 litre, 19.22 US gallon) tank I would use a filter that is rated at least 32 gallons (145.47 litres, 38.43 US gallons) per hour water flow.

  • What should the water level be for an external power filter?

    Answer: The water level should be high enough so that the inlet pipe is immersed; this will prevent the pump from overheating, trying to draw in water that is too low.

  • Why do we use an internal filter in the aquarium?

    Answer: All aquariums need to have the water filtered to remove the toxins but with smaller tanks an internal filter is large enough to keep the toxin levels at a safe level.

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