8.1. Review of Filters

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Author : David Bogert

Published :

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By the simplified version there are many types of filters used for the tropical aquarium which can be scored by approximate number of three inch (7,5 cm) fish the filter will support, media capacity, ease of cleaning, cost, and appearance:

Filter# of 3″ fish crystal clear*Media capacity cubic inchesMedia capacity cubic centimeterCostLooksLeak potential
Cartridge internal04-1065 – 165DecentNone
Cartridge Hang-on-Back05-15100 – 245GreatCommon
Airlift sponge2 – 860-2001.000 – 3.250BadNone
Sponge with pump3 – 1060-2001.000 – 3.250BadNone
Matten corner5 – 8100-1601.650 – 2.600BadNone
Bottom-to-top Hang-on-back filter10 – 20180-2603.000 – 4.250€€GreatCommon
Undergravel with airlift15 – 301000-200016.000 – 32.000GoodNone
Undergravel with pump20 – 401000-200016.000 – 32.000GoodNone
Canister filter25 – 50500-10008.200 – 16.400€€€GreatCommon
Static sump100 – 3002000-600032.000 – 100.000€€€€ **GreatSome
Fluidized sump200 – 6002000-600032.000 – 100.000€€€€ **GreatSome
* Assumes a good filter media (foam, plastic pot scrubbers or K1)
** A DIY sump can be made quite cheaply

*# of 3″ (7,5 cm) Fish refers to the number of three inch fish that can be given crystal clear water with this filter. Ammonia oxidation is much easier than crystal clear water so for ammonia oxidation multiply the numbers by ten to twenty. Note this chart is a very very gross approximation. Also this review assumes a typical 29 to 40 gallon (100 to 150 liter) tank with correspondingly sized filters. And it assumes that an overflow siphon is not visually objectionable while a dirty sponge sitting in a tank is objectionable.

Melanochromis johannii
Melanochromis johannii

Now filters are not as simple as the chart above would suggest. There are many types of filters and the media that they are loaded with must be taken into consideration. There are also many more considerations for a better filter/filter media selection. We have taken the above simplified rating and expanded it to a much more comprehensive look at filters/filter media combinations. This is lengthy and boring but many are interested so here goes.

Rating most of the filter/filter media possibilities from one to ten, with ten being the best, gives the following tables for twenty different filter/filter media combinations:

ParameterAirstone foam/spongeSmall powerhead foam internal canisterPowerhead foam/sponge
Mechanical filtration204
Fry safety101010
Power consumption1077
Water clarity136
Leakage potential101010
Table of sponge filters
ParameterHang-on-back biowheel with undergravelPowerhead undergravelAirstone undergravel
Mechanical filtration1000
Fry safety1*810
Power consumption4610
Water clarity585
Leakage potential61010
Table of undergravel filters
ParameterCanister cheap filled with K1 media, pot scrubbers or foamCanister cheap filled with ceramic rings, bioballs or MatrixFX6 filled with K1 media or foamFX6 filled with ceramic rings and bioballs
Mechanical filtration0000
Fry safety1*1*1*1*
Power consumption2133
Water clarity101107
Leakage potential0100
Table of canister filters
ParameterSump K1 fluidized bedSump sizable hydroponic setupSump static filled with K1 media or foamSump static filled with ceramic rings or matrixSump static waste basket pot scubbersSump static wet/drySump static filled with bioballs trickle
Mechanical filtration101091091010
Fry safety1*1*1*1*1*1*1*
Power consumption1511111
Water clarity1051037610
Leakage potential8288883
Table of sump filters
ParameterHang-on-back bottom to top flow and foam mediaHang-on-back bottom to top flow and ceramic mediaHang-on-back cartridge
Mechanical filtration4210
Fry safety1*1*1*
Power consumption555
Water clarity510
Leakage potential666
Table of HOB filters

* These numbers can be taken to 10 by the simple addition of foam prefilters.

The filter options in pink have low biofiltration scores. Since biofiltration is the most important variable in most aquariums these filters are not the best choice for most aquariums in OUR OPINION.

Note that biofiltration has two different functions. The first function is to oxidize ammonia to nitrate. The second function is to give healthy crystal clear, clean water. Testing says that crystal clear water needs twenty times the volume of filter media than does ammonia oxidation. The pink filters will give good ammonia oxidation but not give crystal clear water.

picture of an aquarium fish Tropheops tropheops Red Top Chilumba
Tropheops tropheops – Red Top Chilumba

Note that all these numbers assume relatively standard set-ups. A standard fluidized bed sump has mechanical filter socks on it. So the mechanical filtration is excellent. I personally use no socks or pinkie pads in my fluidized sumps so my sumps would have a zero for mechanical filtration.

These rating also reflect that 20 and 30 ppi foam is a great biofilter but not a good mechanical filter. Mechanical filtration is only provided by filter socks, poly-fill, polyester cartridge socks, polyester pads (Pinkie Pads) or 40 ppi foam. So the small internal cartridge with 30 ppi foam is a very poor mechanical filter. Since the amount of foam in these tiny units is very small they also do not do very good biofiltration.

In order to understand these ratings one would need to read the chapters below on each type of filter. And understand everyone will rate these attributes differently. These ratings are simply the author’s opinions, something which I normally avoid but there is no way to test all the attributes. Only the biofiltration is based on testing.

Maylandia lombardoi Kennyi
Maylandia lombardoi – Kennyi

The “Best” Filter

The “best” filter is a fluidized bed sump filter with a score of 71. This is the most common type of filter found in large profit driven recirculating aquaculture systems (RAS), such as tilapia or catfish farming. In these systems what hobbyist’s call a fluidized bed (alternatively what the farmers call mixed bed bioreactor, moving bed biofilm reactor or MBBR).

This type of filter is two to ten times more effective at giving ammonia oxidation and crystal-clear water than any other filter on a cubic inch basis (see the chapter on sumps below for six studies supporting this). And it fully saturates the water with oxygen. For aquariums over 55 gallons (200 liter) it is the best filter.

photo of the “best” filter, a fluidized bed sump
the “best” filter, a fluidized bed sump

By the score here an under-gravel filter with a powerhead on it is the equal of a fluidized bed sump (both with a score of 71), based largely on cost. It is basically a “poor man’s fluidized bed”, giving excellent results with very low cost.

Copadichromis borleyi Red Fin
Copadichromis borleyi Red Fin

The “Worst” Filter

The two worst filters by far are the ceramic ring/bioball filled external canister with a score of only 22 and the cartridge hang-on-back filters with the same score of 22. What is interesting is that these two combinations are definitely the most widely used filter setups in the hobby!

Note that, in the article on canisters, we delve into the fact that it is largely the volume of the media and the type of media used which determine the efficiency of a canister filter. It is NOT the flow rate. Also note this low canister rating does not include the newer canisters which have huge volumes of foam in them. They are all excellent filters (ignoring the propensity to leak).

Hang-on-back filters are generally not good filters unless they are paired with a filter with more biofiltration capacity, such as an under-gravel filter. Even the bottom to top flow designs just have too little biomedia capacity for most aquariums.

Now of course every aquarium owner is going to weigh each parameter differently depending on what they want in an aquarium. Twenty small basement aquariums used largely for breeding fish will be weighted considerably differently than a large living room aquarium for home décor. And if you don’t have a lot of money things can become much different.

picture of aquarium fish Labidochromis caerulus Blue Thumbi
Labidochromis caerulus – Blue Thumbi

There are many brands of filter out there and each has its cadre of fiercely loyal followers. Asking which brand is best is like asking a redneck which brand of pickup truck is best: Ford, Chevy or Dodge. But I will make some recommendations:

Recommended Filters

  • Sponge Filters should be super charged by adding powerheads to the air lift tubes. Note if one uses a powerhead on a sponge filter in a breeding tank, make sure the filter is at least five inches by five inches. A smaller sponge can suck the fry to the sponge with a powerhead. And the Aquarium Co-op sponge filters are a very good design.
  • If you constantly clean under-gravel filters they don’t work. And they are a big pain in the butt. But if you just leave them alone, they are a great biofilter at a great price. They should be super charged by adding powerheads to the lift tubes. All my aquariums (15 by last count) have powerhead operated under-gravel filters in addition to other filters.
  • I don’t like hang-on back filters, period. But if you have a hang on back it should be super charged with foam inserts per the directions of Cory at Aquarium Co-op.
  • For small aquariums under 30 gallons (100 liter) I like the Sunsun HW-603B External Canister Filter ($40). This small filter has about 80 cubic inches (1.3 liter) of flow through foam. It can be hidden below the aquarium out of sight. It will give excellent biofiltration and crystal clear water if the instructions to change the foam every four months are ignored.
  • For aquariums from 30 to 75 gallons (100 to 300 liter) I like FX canister Filters. The foam in this filter and the way the flow goes (down-up-down-up) is just a superb combination. But I do replace their hoses. And I definitely pay no attention to the ridiculous profit oriented “recommended maintenance replacement schedule” that comes packaged with the filter. The foam media will last for the life of the filter and replacing it will actually reduce the capabilities of the filter.
  • Many million dollars of research in recirculating aquaculture systems (RAS) has clearly established that two or three chamber K1 Fluidized Bed Sumps are two to ten times better on a cubic inch per cubic inch basis than any other filtration system. I use easily built cheap 40 gallon (150 liter) fluidized beds with cheap K1 rip offs from China in all my tanks over 75 gallons (300 liter). And the water is crystal clear and well oxygenated in all of them.

Note that I love under-gravel filters and use them in all my aquariums. I’ve used a ton of different hang-on-back and canister filters from a lot of different manufacturers and I find that leaks and blown motors are very common across the board. And they can occur out of the box! An under-gravel filter is so simple it can’t break. I’ve got operating under-gravel filters that are forty years old. Canister and hang-on-backs only have a lifetime of five or so years if you are lucky. I like to pair up each under-gravel with another filter, either a canister or a sump. I now use NO hang-on-back filters on any of my tanks.

picture of an aquarium fish Labidochromis caerulus Yellow
Labidochromis caerulus Yellow

Filter Ammonia Oxidation Test Results

Ammonia was added to aquariums with various filters. The resulting ammonia levels were measured three days later, and the amount of ammonia added adjusted according to the measured ammonia level. The test went on for 90 days. These are the results:

FilterAverage15 days30 days45 days60 days75 days90 days
1 inch of gravel0,50.250,50.50,2510,5
Cartridge HOB3,8142484
Sponge Filter84481688
Bottem flow HOB184832163216
Static sump8183264128128128
Fluidized bed sump131166412864256256
Milliliters dilute Ammonia solution oxidized

The “average” number represents the number of four inch fish each filter can give crystal clear water with. The number of four inch fish that can have their ammonia oxidized is about twenty times this number. Ammonia oxidation is much easier to obtain than crystal clear water.

Note that the “One Inch (25 mm) of Gravel” in this test fairly well puts to rest the idea that the substrate in an aquarium acts as an ammonia oxidizer. This idea is a myth, per this test. Note that 0.5 is the ammonia that a few three or four inch fish would put out, so it is possible to have an aquarium with no filter in it and no ammonia. It’s just that the stocking must be very light. A gravel substrate is a very weak biofilter.

This test can be reviewed at this link:

8.1.1. Filter Test

picture of an aquarium fish Labidochromis hongi
Labidochromis hongi

Deep Gravel Bed

There are some aquarium YouTubers who advocate something called a “Static Deep Gravel Filter” as a “Natural” approach to an aquarium. It is possible to slowly over time build up a population of plants and fish with only three to five inches (7.5 to 12.5 cm) of gravel in the aquarium. The gravel accumulates a heavy biofloc which ties up the ammonia, nitrite and nitrate as biofloc.

This becomes an aquarium where the surfaces on the plants, detritus and the gravel act to accommodate ammonia oxidizing bacteria. Typically the water is filled with small particles and is far from crystal clear. And one dead fish or some over-feeding will present some problems to the tank. I do not like this sort of aquarium. But one can read more about it at this article:

15.11. Lots of Plants, Lots of Fish

Further Data

The following sections try to explain the different choices one can make in far more depth:

8.2. Hang-on-back Filters

8.3. Canister Filters

8.4. Sponge Filter

8.5. Under-gravel Filters

8.6. Sumps

8.7. Other Types of Filters

8.8. Sizing Filters and Media

8.9. Anaerobic Reactors

Chindongo elongatus cf. Chewere
Chindongo elongatus cf. – Chewere