So, you are choosing the best canister filter? The most important fact you should know is that their manufacturers quite often play hanky-panky with you when indicating the amount of water being pumped by a filter per hour.
So, in this respect you should buy a bit oversized filter. And this is it, now you may skip the rest of the article since below there are only boring details and comparisons, characteristics and IMHO as for studied canister filters.
However, if you are interested you are welcome to continue reading…
Aquarium filters you may encounter quite often are those you can buy at Amazon.com
In general, I’ve listed all worthy filter models and if there are others, then they are made by some noname brands, or they are some rare models, to describe which I haven’t found sufficient amount of information.
If you are in a hurry and have no time to read about all tank filters I’d recommend you to pay attention to these ones:
- 1 Facts you should know
- 2 Canister filters review
- 2.1 EHEIM Classic canister filters – 20 years on the market
- 2.2 EHEIM Professional 3 canister filters
- 2.3 EHEIM Ecco Pro – energy-saving filters
- 2.4 Fluval Fx6 and Fluval Fx4: they are for cichlids and other large fishes fans
- 2.5 Fluval 406: just a good filter
- 2.6 Fluval G6 Advanced Filtration System: for fans of marine tanks or new technologies
- 2.7 Marineland Magniflow Canister: beginners friendly
- 2.8 Hydor Professional External Canister Filter: Great quality for the money
- 2.9 Sunsun canister filter: a budget choice
- 2.10 Finnex PX-360 Compact Canister Aquarium Filter: for small tanks
- 2.11 Rena Filstar Canister Power Filter: the mystery
- 2.12 Aquatop CF Series Canister Filter: typical “made in China”
- 2.13 Penn Plax Cascade Canister Aquarium Filter: Under 75$
- 2.14 Cobalt Aquatics EXT aquarium canister filter: the most unusual technical solution
- 2.15 Zoo Med 511 Turtle Clean Canister Filter: for turtle fans
- 3 The rest of filters: it’s difficult to tell
- 4 So what is it exactly – biological, mechanical and chemical filtration?
- 5 Now It’s Your Turn…
Facts you should know
I assume you’re quite busy guys and you don’t have time to read all of this and enter into all the details.
Well, me either.
Then, here are the facts you should know before paying your money for the filter:
There is a rather wide range of external filter brands and there are much more filter models on the market.
What is confusing that famous and old brands have a lot of different filter models – easy in care and cheap ones, expensive and sophisticated ones.
If you want to buy a filter for your tank and you see one or three models provided by the brand, it means that the company isn’t focused on filter production or maybe it has just entered the marked.
In my opinion company’s specialization is important since it is a determinative factor and those who produce lots of stuff in small amounts loose to those who are focused on production of just one product.
Canister filters review
EHEIM Classic canister filters – 20 years on the market
These filters can be used in any tank, they are plain and sustainable. If you don’t want to get into details and just to buy a filter and turn it on, this is the one you need.
Its drawbacks are that it comes short of filtration rate and some other abilities when comparing to more modern models and you’ll have to buy filtration media for it.
Multi-function models: Eheim 2217 (US gal/h: 264 gph; power consumption 20w) and Eheim 2215 (US gal/h: 164; power consumption W: 15). Despite the fact that «Classic» filters product line has been on the market since 80s it has filters justly called a “willing horse”, some of them have been used for almost 20 years.
One may say that these were classic filter models – Eheim 2215 and Eheim 2217 that made Eheim its good name. EHEIM Classic, sold at Amazon.com, is equipped with filter media (EHEIM Mech & Substrat Pro).
Here is the filter with the simplest design.
It can be considered as a straight pipe filled with filter media.
Tank water gets to the filter bottom, comes through the filter media and then it is already clean and it’s pushed out by the pump through the other end of the pipe (its upper side) back to the tank.
Filter media in this filter type can be placed both directly in its canister and is a special cassette made for it. When installing the cassette you should push it down harsh but slightly till it is fixed in its lowermost position.
- It can be used in any tank
- The filter quality has been proved by years of their usage
- Several such filters can be used in a big tank
- Small internal volume is compensated by its maximum efficiency
- Very low power consumption
- Reliable ceramic axel and bearing part
- Some filter sets have only sponges without fillers
- Some models don’t have double taps
- Small internal volume
- Old-fashioned design
- Manual prime
- Rather fragile plastic
- Filter set doesn’t have any means to simplify the priming Process or filter maintenance and there is no starting bacterial culture
EHEIM Professional 3 canister filters
They are one of the most modern canister filters, they will do for those who prefer maximum quality and comfort. There is some confusion among the filter models, but I’ll explain.
By means of USB cable the filter is connected to computer and it’s getting easier to control it, though the USB cable you should buy yourself.
In «Professionel 3» and «Professionel 3 electronic» external filters product line the most popular are EHEIM 3e 2076 (up to 105 US Gallons, 435 gph, 35 watts)
All of them have the shape of elongated cube with a canister divided into sections, filter head with a pump and water preliminary treatment filter in it.
For those who decided to really spend money on the filter I’d recommend to take a look at Eheim 6666 Pro 4+ Canister Filter, the latest model that has just appeared on the market.
There are 3 models you can purchase, the 250, 350, and 600. The 250 is for 30 – 65 galllons, the 350 is for 50 – 95 gallons, and the 600 is for 65 – 160 gallons.
It has Xtender function in it, due to which you can clean the filter more seldom.
If the filtering media gets clogged with dirt the flow is redirected and biological filtration efficiency is preserved.
Filter models 250 and 350 can also have an integrated heater as thermo filter (T), at that the company has taken into account aquarists’ feedback and now the heater works separately from the filter.
- Long hoses
- German quality
- All supplementary stuff is present
- Spray bar
- Noiseless and vibration-free operation
- High quality of tank water filtration
- High flow rate at low power consumption
- Easy connection and priming
- Reliable leakproofness
- The price
- Poor technical support in North America
- It’s difficult to find spare parts
- The filter set doesn’t have any means to simplify the priming process or filter maintenance and there is no starting bacterial culture
EHEIM Ecco Pro – energy-saving filters
This is a product line for those who care about ecology and don’t like paying enormous electricity bills.
The filter energy consumption is 8 Watt at stated by the company filtration abilities equal up to 60 US Gallons and it is 5 Watt at 35 US Gallons correspondingly – these are very nice numbers.
- Compact size
- Economical efficiency
- The filter is equipped with filter media
- Cleans tank water from fine suspended solids very well
- Handy fixation of baskets to each other
- Priming and maintenance friendly
- Convenient carrying handle
- Sintepon gets dirty very fast
- Absence of spray bar
- ill-considered pump fixation – due to this leakages are quite possible
- High price
Fluval Fx6 and Fluval Fx4: they are for cichlids and other large fishes fans
The company has its best-in-class filter, a powerful filter FX-6 for tanks up to 400 gallons (filter output 563 GPH, 41 Watt).
This filter is made for tanks with large amounts of organics that has to be removed mechanically.
These are large tanks with plants or with large fish. Fluval FX 6 also offers a “Smart Pump” technology which ensures the filter maximum output.
The filter has an electronic circuit that continuously controls its operation. Impeller spin speed and rolling force are measured continuously to ensure the filter output and efficiency.
The circuit also provides the filter autorun and removes the air which can accumulate inside the system. Once in 12 hours the pump stops and lets the accumulated air out of the system ensuring by this the best filtration efficiency at any moment of time.
A new smart-pump has been designed for Fluval FX4, which provides the filter maximum output.
The pump has a special electric circuit inside which completely controls its operation. It controls the pump power and rotor speed thus it ensures filter maximum output and power consumption efficiency.
All filtering details were thoughtfully chosen. The filter consists of 4 containers with total filtration volume (14L or 3/7 US Gal) and biological volume 3.9 L (1 US Gal).
Filter baskets come in order in such a way that it provides maximum efficiency and crystal clear water in a tank. Filtering medium and fillers are put in several layers so the water when coming through the filter gets maximally clean from dirt, garbage and harmful microorganisms.
The baskets with fillers have subsidiary filtering sponge on the edges which performs additional mechanical water purification.
Each basket is responsible for efficient water filtration; foam performs mechanical, preliminary water purification by capturing the smallest garbage particles.
Fluval FX4 filter has a special handler by means of which you can take out and separate all baskets easy and fast.
Such an improvement was designed to simplify maximally the filter maintenance. Now you don’t have to waste time disassembling the filter completely to wash the fillers.
There is an indicator on top of the filter which helps you to remember the date of the last maintenance.
- Large volume
- Large amount of filter media
- Monthly maintenance indicator allows remembering the date of the last technical service and water renew
- Pump undermount
- System with drain hose which allows to drain water and fill with it without disassemble ing the system
- It will do both for freshwater and marine tanks
- Autorun function
- “Click-Fit AquaStop” taps ensure fast and reliable maintain the filter
- Bottom pump may get clogged with sand and then jam
- The filter is made for mechanical filtration and the biological one takes a backseat
- Rather expensive
- Not maintenance friendly
Fluval 406: just a good filter
Fluval 406 Flow Rate: 1450 L/H (383 US GPH) – this is a legendary improved external canister filter of the 6th generation from Hagen.
This filter is for optimal water filtering in freshwater or marine tanks up to 400 Liters (100 U.S. gallons).
- Its engine can provide strong water flow through filter media
- Unique filter design ensures multi-stage filtration
- Almost any filter media can be used for 06 model
- Aquastop system prevents water leakage since it turns the filter off once the hoses detach or when the filter gets clogged
- By means of the new clamps you can easily create a filter and reliably fix the engine
- Due to the fact that the filter sponges are easily accessible it’s not a problem to replace the filler
- Using fast start system it’s simple and easy to prime the filter
- 3 years warranty
- Canister small capacity
- Small amount and confusing set of filter medium (you’ll have to buy some yourself and replace them)
- Only branded sponges can be used
- Very small amount of ceramic filler in a set
- Canister plastic is very thin
- Fast start system is rather slimpsy
- Rotor ceramic axel is thin
- Rotor blades are also quite thin
- It has only two clamps
- The baskets are rather small and thin
Fluval G6 Advanced Filtration System: for fans of marine tanks or new technologies
Fluval G6 will do for those who are keen on technologies, for whom their filter appearance is an important part of interior environment and also for those, who love marine aquariums.
This is an innovative, highly-efficient and quiet external filter of G-series, which ensures three stage filtration of tank water and this is an ideal choice for marine and freshwater tanks.
It was stated that despite the small output the filter can easily deal with a tank up to 600 liters, however in the real life you should halve this number.
The filter rate is 265 gallons per hour.
The main innovation in Fluval G6 filter – is «Hydrotech Performance Monitor» module – a set of sensors that measure temperature, flow rate and conductivity.
As well as a processor unit with display that ensures both managing data of current measurements and then gather data and output it onto interactive LCD-display.
Fluval G6 monitor allows controlling temperature, water flow rate and conductivity, and it displays all these parameters as numbers and graphs.
The filter software continuously monitors the filter functioning and in case of emergency displays the warning message ALERT.
The filter monitoring system also reminds its owner to the perform maintenance service when it is necessary.
- Servicing schedule (displays information about the necessity of filter maintenance (to replace its cartridge)).
- Water conductivity chart (shows conductivity fluctuations and informs about tank water chemical composition change).
- Temperature chart (displays tank water temperature changes).
- Alert information display (indicates when the parameters differ from the given ones).
- Stylish design lets you put the filter as a part of interior environment; you don’t have to hide it somewhere
- Maintenance friendly
- Embedded software to control the filter main parameters
- Very quiet in operation
- There is no statistics and setup data loss when the filter turns off
- Innovative filtration system
- The following data is displayed: temperature, water flow rate (if it gets low an alert appears that the filter requires washing), the display shows water electrolytic conductivity (when this value rises it’s a signal to renew the water)
- For marine tanks it displays salinity degree
- Quality fillers
- Easy to clean – you don’t have to open the lid completely
- Well equipped
- Not very high
- It will do well for marine tanks
- The price
- Expendables are to be bought and replaced regularly
- Expendables price
- It won’t do for tanks with large fish, since you’ll have to replace expendables very often
- The filtering media gets clogged very fast and they have to be replaced
- A bit confusing fast start system
Marineland Magniflow Canister: beginners friendly
Marineland Magniflow Canister is produced by an American company and has proved itself as a reliable and working system.
It’s meant to be used for volumes 30 (160 GHP), 55 (220 GHP) and 100 (360 GHP) gallons, it can perform mechanical, biological and chemical filtration.
- Very quiet functioning
- Positive reviews at Amazon.com; the majority of filters work for about 5-7 years without any troubles
- Plain and easy filter priming
- Moderate prices for expendables and spare parts
- 3 years warranty
- Perfect users support
- Reasonable price
- It is easy to disassemble and clean
- No leakages
- Water intake pipe has fixed length so it may not fit all tanks
- No spray bar in the filter set
- Rather fragile plastic is used
- Some filters have noisy impeller and this noise is rather difficult to eliminate
- O ring has to be greased with silicon since it’s not airproof
- The filter system clogs very easily, slowing the flow to a mere trickle
- A lot of bypass
- All of the plastic hose clamps have fallen off of the hoses
Hydor Professional External Canister Filter: Great quality for the money
It had Hydor prime products line and now Professional model has come to substitute it. The new product line called Hydor Professional External Filter consists of 5 models (for tanks from 20 to 150 gallons); they have different pump output which allows choosing a model required just for your tank.
Canister of Hydor Professional external filter is made of sturdy plastic, it has rectangular cross-section which lets you easily put it in the corner of your tank stand.
The canister clamps have a system that prevents their self-opening and thus ensures leakproofness and reliability of the tank filter. All filter media and fillers come in the filter set.
Each component is in separate basket which allows taking out each of them easily from the filter canister to wash or replace.
Tight baskets arrangement in the filter canister provides good water flow directly through the filter media, which prevents clean and dirty water mixing inside the filter.
Top Hydor Professional 600 designed for tanks 90-150 gal has the following parameters:
- Filter circulation: 290 GPH;
- Pump output: 345 GPH;
- Power consumption: 35W;
- Filter volume: 12.9 qtr.
- It is good for marine and freshwater tanks
- Ceramic axle ensures failure free filter operation
- It has universal large volume filter baskets
- The filter maintenance is performed fast and easy, the hoses can be easily detached
- Due to the reliable clamps and special o ring any leakage is prevented
- The filter is supplied with all necessary fillers excluding carbon
- It has a sectional spray bar that consists of several sections
- Wide, conveniently fixed hoses
- Easy assembling and fast filling with water
- Quiet operation
- Very large priming button
- Extensible tubes
- Large internal volume
- It doesn’t have any spare parts, so if something gets broken when out of warranty it’ll be difficult to find a replacement
- It’s desirable to buy some more filter media, since the filter doesn’t have rather wide range of it by default
- The filters are brought from far, so sometimes they come with damages because of this
- Not the best quality plastic is used
Sunsun canister filter: a budget choice
Sunsun company (China) produces one of the simplest and cheapest filters among all of the presented models.
The company has rather wide product line, but all filters have common characteristics and list of equipment. These filters are in great demand, since quite often price plays a crucial part.
But are they really as cheap as they seem? Let’s find out.
For example, let’s consider SunSun Hw304B 525GPH Pro Canister Filter Kit with embedded UV-sterilizer for tanks up to 100 gallons and pump output 370gph.
UV-sterilizer power capacity is 9 W and it provides crystal clear tank water preventing appearance of green algae, different microorganisms and pathogenic bacteria in a tank.
So, you can turn of your UV lamp. The filter has high quality ceramic axel to ensure its noiseless operation.
There are 4 baskets for different filter media – biological, mechanical and chemical one.
The filter is equipped with:
- 9 watt UV
- 3 Pack of EXTRA White Filter Pads
- 1 Pack of Blue Coarse Pad
- 1lb. of Premium Filter Carbon
- 1lb of Ceramic Rings
- 40 Pieces of Bio Balls
Sounds good, isn’t it?
UV plus expendables….
However, in fact all of them are of rather low quality and their amount isn’t enough for the filter to function efficiently. Some models are sold even without all of the stuff mentioned above, they only have sponges.
- The filter has UV sterilizer, though it’s not powerful enough to operate efficiently
- The sterilizer comes first and all tank dirt will be accumulated on it, since it has to be placed after filtration
- Rather low price (there are no analogous filters with the same parameters and the same price)
- Very noiseless in operation
- It has a pump for priming
- Rather fragile stop valve (when you turn off the water it seems that it is about to break in your arm)
- Clamps aren’t reliable
- No spare parts
- White color is very easily soiled
- It’s necessary to buy expendables
- Poor engine performance – filters with the similar output produce twice as strong water flow
- Short hoses
Finnex PX-360 Compact Canister Aquarium Filter: for small tanks
Finnex PX-360 Compact Canister Aquarium Filter is for 5 to 20 gallon aquariums, its output is 90GPH, power consumption 11.5w.
It can be recommended for nano tanks, since its low price allows using it in small capacity tanks and its output isn’t enough to create a strong flow.
However, you have to keep in mind that 20 gallon tank is too large for such a filter, so better use it in smaller ones and for large tanks a more powerful canister filter should be bought.
The Finnex fiter is suitable for 5-10G tanks and if a tank is larger the filter may fail and you’ll have to wash it every week.
- Very quiet
- The price
- Super easy to clean
- Doesn’t create a strong flow (90GPH)
- It is supplied with a spray bar
- You can adjust outflow
- Optional sponge on the intake
- Large internal volume
- Super easy to maintain and prime
- Lots of pieces to customize
- Short hoses
- Low power for efficient biological filtration
- It’s advisable to replace ceramic parts with more efficient materials
- Too bulky
- Needs washing too often
- Instruction for assembly is horrible. Make sure you double check with minor leak after you first assemble. Any tiny leak indicates using the wrong parts.
- With active carbon/sponge, flow rate slows down after one month of use in heavily planted tank.
- The baskets are made of very fragile plastic, you should be careful with them
- It has no priming button, so you have to fill the filter with water manually for the first priming
Rena Filstar Canister Power Filter: the mystery
Until recently these filters were produced by another company and they were quite good. Now it has been bought by API and the production has been transferred to China.
The company has a product line API FilStar XP. Here after XP the model is indicated according to its output. For example, XP 4 filter. One of the most popular models in this line is Rena filstar XP-L (former Rena filstar XP 3).
This is an external canister filter for tanks up to 175 US Gal, output 350 US GPH, power consumption 19 watts. Unfortunately, this model has got negative reviews at Amazon.com.
First of all the buyers complain about low quality of plastic and leakages.
Two years warranty is good, but if you use the warranty coverage several times it’s annoying.
IMHO I’d be very careful when choosing this filter and I’d rather wait for the company’s response to the complaints.
- 2 years warranty
- Very quiet
- Large internal volume
- Good quality filter media
- Easy priming
- No extra charges for the brand
- It has no spray bar
- More frequent complaints about leakages
- Low popularity at amazon.com
Aquatop CF Series Canister Filter: typical “made in China”
Not expensive Chinese filters for those who don’t want to spend a lot and don’t care much about maximal water purification.
This filter is represented in three models that differ in output value, filtration rate and US-sterilizer presence.
The earlier model Aquatop CF300 is made for fresh and saltwater aquariums up to 75 gallons and its output is 264 gals/hr.
Aquatop cf400uv model is for tanks from 75 gallons to 125 gallons and its output is 370GPH, it has UV-sterilizer as well and its power capacity is 9 W.
CF500UV model also has a sterilizer and its made for tanks up to 175 gallons and it pumps 525 GPH.
- Its price
- Any sterilizer is better than none
- It’s good for tanks with turtles and not demanding fishes
- Sterilizer power isn’t enough for such a filter
- Not sufficient amount of filter media, you should buy some more to ensure the filter efficient operation
- It doesn’t have the flow control
- White color
- Made in China
- Quite frequent leakages
- It isn’t suitable for tanks with demanding fish species or for very crowded tanks
- Very hand-exhausting difficult assembly and set up
Penn Plax Cascade Canister Aquarium Filter: Under 75$
It’s a simple, reliable and use-proved filter.
Penn-Plax Company is a popular American manufacturer of cheap goods for aquariums. Filters produced by Penn-Plax are manufactured in China, however they are rather good quality ones.
Penn Plax Cascade product line is represented by canister filters for marine and freshwater tanks from 30 US gallons to 200 US gallons.
Thus Penn Plax Cascade 1000 canister filter is for aquariums up to 100 gallons and pumps 265 gallons per hour (GPH). There are two trays with foam in each one and also charcoal in one and course filter media in the other. The filter is equipped with a spray bar.
- It’s equipped with a spray bar
- It has a button for water injection that eases the filter priming
- Price is very low when comparing to other branch
- You can put different filter media into the large trays with handlers that can be put on top of one another
- Very quiet operation
- Fast and easy hoses detaching for maintenance
- Super easy to setup
- Easy to clean
- Just like all other companies this one also states the filter output value to be larger than it is in fact
- Pipes are made of fragile plastic
- It’s hard to prime
- Cheap filter media and cheap baskets
- The carrying handle is very flimsy
- Written instructions and pictures are simply not accurate
Cobalt Aquatics EXT aquarium canister filter: the most unusual technical solution
The filter is good for those who don’t have the ability to put a filter under a tank.
This filter’s feature is that it doesn’t need priming and it can be put below, next to and even above the tank! Since it just doesn’t have a pump in its head, the pump is put into the tank together with the intake pipe.
This allows switching of the filter and washing it without turning the pump off. I.e. you turn the latch handle, take away the canister and the pump continues working.
Max flow rate is 210 gph (800l), the filter is for tanks up to 55 gallons, total canister volume is 1.85 gals.
Complete with four pre-loaded media chambers
- 1st stage sponge filter
- 2nd stage activated carbon
- 3rd stage ceramic rings
- Final 4th stage finishing floss
- 5/8″ ID hose with plastic snap clamps
- Unusual, even unique technical decision
- Very low price
- Ability to maintain the filter without switching it of
- No need in priming
- Canister itself is very sturdy
- Location of the pump outside the canister allows increasing its effective volume
- Three years warranty
- Quick disconnection of valves which allows the water to keep flowing
- Tons of included media
- The filter box is designed to have little or no bypass.
- There is only one filter model in the product line – just for tanks up to 55 gallons
- The pump on the intake pipe consumes additional space in a tank and it’s far more noticeable than a common pipe
- Cheap accessories
- External pump adds more complexity to the setup
- The included plumbing kit is unusable
- No spray bar. This type of filter system generally works better with spray bar to help break surface tension and provide the water with oxygen exchange. Without something in an aquarium to provide this function the fish will suffocate and die.
- The intake screen doesn’t fit any of the tubing adapters and will barely stay connected to the pump. For internal pump configurations the screen needs something to affix it to the pump. For external pump configurations you are pretty much screwed.
- Polishing floss is just an unruly mass of loose fluff
- No clamps for hoses either at the canister or at the u-tubes
- No way to secure the intake and outflow hoses from sliding around
Zoo Med 511 Turtle Clean Canister Filter: for turtle fans
Have you ever had a turtle in a tank?
Water in a tank starts to smell just in two days and washing this tank is rather doubtful pleasure.
Zoo Med 511 canister filter was made especially for tanks with turtles – it is supplied with a spray bar for better water aeration. Besides turtles like lying in water flows that come out of the spray bar.
The filter is made for tanks up to 60 gallons. But this is a manufacturer’s trick – in fact the filter will only do for a tank up to 30 gallons.
- Low price
- The filter is extremely easy to disassemble and clean and its fairly user friendly
- Good technical support
- In fact the filter is for tanks up to 30 gallons, but not for the stated 60 gallons
- Fragile plastic used. If you choose to purchase this, use extreme care when manipulating the poorly manufactured caps.
- Frequently breaks
The rest of filters: it’s difficult to tell
There are some external filters which aren’t very popular so it’s rather difficult to give any feedback. First of all because they are produced in China and even their manuals are written in Chinese.
So what is it exactly – biological, mechanical and chemical filtration?
• Biological filtration, i.e byproducts transformation in a tank. This involves deactivation of ammonia and ammonium which are toxic for tank inhabitants and transferring them into less harmful nitrates that in turn are partially recycled by tank plants and partially removed when performing water renews.
Biofiltration is performed by nitrifying bacteria that inhabit in a filter.
Technically speaking any filter is more or less occupied by nitrifying bacteria and it can be called a biological one. The thing is that biofiltration is just more efficient in filters of larger volume than in smaller ones.
• Mechanical filtration or cleaning the water of organic or inorganic suspended matter.
Many people consider that this is the main purpose of any filter. Of course it’s not very nice to see muddy tank water.
However, the main operating result of a filter is still an efficient biofiltration, not just cleaning tank water of suspended matter and mud. In case of well arranged biofiltration process the tank water is crystal clear.
• Chemical filtration is based on filter medium and tank water interaction that changes the water properties or content of some matters in the water.
Ionite, zeolite and other substances are used as a filter medium.
• Adsorbtion of some substances (for example some medicines after the treatment was finished) this is when carbon is used.
The larger is a ratio of filter media to a tank volume the more efficient filtration is. It’s rather difficult to put a large filter in a tank, since it’ll take additional effective volume.
Such internal filters are the best for small or sparsely populated tanks. In the rest of cases an external filter should be chosen.