Aquarium Canister Filters

8.3. Aquarium Canister Filters

One Big Scam

Any discussion of ANY filter requires a disclaimer. ALL commercial filters almost without exception tell the owner to replace the filter media (Sponge, ceramic rings, carbon, Matrix, etc.) on a regular basis, once a month, once every three month or once every six months. Supposedly this is because the media becomes clogged and dirty. And the media is ridiculously expensive. We must emphasize:

Yes, there is absolutely no reason to replace the media on a regular basis other than to make a lot of money for the supplier of the filter. And be aware that activated carbon has only a few very limited uses in some aquariums and is a totally unneeded media in 99% of all aquariums.

Mylochromis sp lateristriga makanjila Hai Reef
Mylochromis sp lateristriga makanjila Hai Reef

One Caution

I have had several canisters fail and leak, And I’ve done some big time damage to some hardwood floors as a result (one pissed off wife!). So I am NOT a big fan of canisters and only had a few of them in rooms with concrete floors.

A canister filter requires a siphon overflow on the aquarium. It is important to either position the siphon so that the inlet is only an inch or two below the surface of the aquarium water OR put a hole in the siphon one or two inches below the surface of the water. This way you limit the damage from a canister leak. One can also put the canister in a tub or tote to catch leaks.

Put the canister in a tub large enough to hold the one or two inches of water. This way the siphon will stop working if the level of the water gets even a little low. This may well result in the filter impeller burning out and needing replacement but that is a lot cheaper than replacing flooring. You can also add a water sensor alarm to the tub. You can buy small water sensor alarms on Amazon. They are a cheap insurance policy.

Another point with canisters is that one needs to be cautious about filling the baskets. Do not overstuff the baskets. If the baskets can’t seat well against each other they can rattle and significantly increase the noise level coming out of the canister filter.

Neolamprologus pulcher Daffodil Cichlid
Neolamprologus pulcher Daffodil Cichlid

What are Canister Filters

Most canister filters are pressurized closed vessels which had a series of baskets that the water is suctioned through, the top basket first (downward flow through the baskets). A few canisters do a bottom up flow. And some do side to side flow. The first basket might be filter floss, the second basket bioballs, the third basket activated carbon, and the fourth basket ceramic rings.

aquarium canister filter
aquarium canister filter


There is one simple truth when it comes to canister filters:

A SunSun 303 canister filled with only 20 ppi (pores per inch, i.e. coarse foam) foam can give crystal clear water with 35 five-inch Mbuna. The same canister filled with ceramic rings, BioHome or lava rock can only give crystal clear water with 4 five-inch Mbuna. These huge differences were confirmed by testing.

Neolamprologus tretocephalus
Neolamprologus tretocephalus

A canister filter is a powered filter that costs about $40 to $500. They typically pump 200 to 700 gallons per hour (GPH). Note that the pump in a canister is rated in GPH at no filtration media in the canister. Add the normal amount of media and the rated flow can be cut by half. Also, the cheaper the filter the more inaccurate the GPH claims.

That being said the common myth about canisters is that it is the flow rate which is important. That is a myth. It is the volume of the filter media and the type of filter media which is important. ANY reasonable flow rate with two gallons of pot scrubbers will successfully filter close to twice the fish load of one gallon of pot scrubbers.

The concept that flow is important was tested. It showed doubling the flow rate added some 9% to the filter capacity. All the well meaning but ill-informed commentators on social media say it should double the capacity. This test can be found at this link: Canisters in Depth

Aulonocara stuartgranti Chilumba
Aulonocara stuartgranti Chilumba

Filter Media Progression

“Mechanical filtration” is somewhat misunderstood. “Mechanical filtration” is a fine mesh filtration media which catches all of the large particles on it’s front surface in a thick brown film. “Mechanical filtration” media includes floss (“Polyfil”), Pinkie pads and 40 ppi foam. The mechanical filter media needs to be the first filter media the water flow hits when it enters the filter. But after the mechanical filtration, the “progression” in pore size is immaterial. ANY media size progression or order after the mechanical filtration will work well.

If one doesn’t use mechanical filtration (I’m too lazy, I do not want to have to open my canisters every week to clean out the mechanical filtration) then the “progression” becomes totally immaterial. I use 100% pot scrubbers with no progression. There is no need for either a variety of media or a progression in the media.

Macropodus opercularis Paradise Fish
Macropodus opercularis Paradise Fish

Canister Filter Media

If canister filters are filled with the biomedia normally used (bioballs, activated carbon and ceramic rings) they do not function well. This type of canister, regardless of its volume or flow, is a very poor biofilter.

If the bioballs, activated carbon and ceramic rings are all replaced by carefully cut foam inserts the canister filter becomes an awesome biofiltration machine. Foam has ten times the biofiltration capacity of most other media. K1 media used in a canister as a submerged static media is also an excellent option as are plastic pot scrubbers.

Ben Ochart has one SunSun canister filter filled with only foam on one of his aquariums. And the water in that aquarium is crystal clear!

Nimbochromis fuscotaeniatus
Nimbochromis fuscotaeniatus

More In-depth Analysis

A test was run on the ammonia-oxidizing capability of various filter media. The first number is the average ammonia oxidizing that 15 cubic inches of media accomplished over a 90-day period. The second number is the effective surface area in square feet per cubic feet calculated by simple math. The correlation between the test results and the calculated surface area is very significant and means the testing was accurate.

from two tests
surface area
Fluidized K1 medianot tested540na
30 PPI foam17340400
Pot scrubbers1428080
Static K1 media13260200
20 PPI foamnot tested220180
Aquarium gravel6120120
Blue Matala pads5100120
Eshoppe bioballs510060
¼ to ½ inch lava rocks36060
Biohome ultimate24030
Ceramic rings24040
Aquarium Filter Media Test and Efficiency

The higher the numbers here the better the media. This makes 30 ppi foam the best media and ceramic rings the worst media. Note foam has to be EXACTLY cut to just the right shape to be effective (can’t have flow around the foam) so  it is easier to just fill the trays in a filter with static K1 media. Plastic pot scrubbers would be a very good alternative obviously.

The details of this test can be found at this link:

7.1.3. Test of Filter Media

Aulonocara korneliae Blue Gold
Aulonocara korneliae Blue Gold

This test was for ammonia oxidation. crystal clear water is a completely different matter. As a very rough average crystal clear water requires twenty times the surface area of ammonia oxidation. This topic is delved into in some depth in this link:

6.4. Crystal Clear Water

What this can translate to is illustrated by the number of fish a typical canister can give crystal clear water with when stocked with some of these media:

  • Foam

            35 five-inch fish

  • Static K1 media

           20 five-inch fish

  • Ceramic rings, Matrix

           4 five-inch fish

These are some huge differences.

Maylandia aurora
Maylandia aurora

Practical Application

What this means from a practical standpoint is that there are two “easiest” and “best” media to fill a canister with. This is to fill a canister 100% with one or the other of the following media:

K1 media for an aquarium filter
K1 media for an aquarium filter

A typical large canister will need three gallons of static K1 type media OR

Plastic pot scrubbers
Plastic pot scrubbers
As many plastic pot scrubbers as one can fit in (in my case 40)

I don’t use foam as it is too difficult to get a good fit in the canister. If you can buy precut 20 ppi or 30 ppi foam for your canister I heartily recommend that for a filter media. If you want to cut your own foam I do not recommend a cheap foam. You can’t tell how many pore per inch (ppi) this foam has nor whether or not it is suited for the aquarium. Rather go direct to Poret Foam supplier and get their 20 ppi foam.

Poret Foam Sheets
Poret Foam Sheets

Mechanical Filtration

Many hobbyists chose to do mechanical filtration in their canisters. Mechanical filtration is actually only done by floss (Polyfil), polyester fiber pads (Pinkie Pads) or 40 ppi foam (Mythbuster: 20 and 30 ppi foam is a biomedia, not a mechanical filtration media). Using one of these three products as the first filter media will strain out particles and give mechanical filtration.  If one of these three products is used in the canister, the canister typically needs to be opened once a week and the mechanical filtration media replaced or cleaned.

Many hobbyists (including the author) do not utilize mechanical filtration at all. For this “uncleaned mechanical filtration” one need only add only one media in a canister doing only one thing, namely biological filtration. That way a hobbyist only needs to open their canisters every six months or so, when the flow slows down. This is explained in this link:

6.1.2. Uncleaned Mechanical Filtration

Note: many people use a whole lot of different media in their canisters. One enterprising chap had 6 different media in his canister. This is just rather humorous as everything except the mechanical filtration media does only one thing, namely biological filtration. Each media is just more or less effective than another media at the task.

Maylandia estherae albino zebra
Maylandia estherae albino zebra

When to use Canisters

The author uses canisters (and under-gravel filters) for all his aquariums in the 30 to 50-gallon range in the grow-out room (concrete floor with linoleum on it). With the larger aquariums over 70 gallons, he switches to fluidized bed sumps (and under-gravel filters) and under 30 gallons he uses sponges (and under-gravel filters).

Note that canisters do not do well with drilled tanks. They only work well with siphon feeds out of the aquarium. An overflow feed doesn’t have the “head” that a siphon has and it causes the weak pump in a canister to struggle. This can severely reduce the flow. Any drilled tanks need to have the drill hole plugged in order to use a canister filter.

Astronotus ocellatus Oscar
Astronotus ocellatus Oscar


Some hobbyists like to use sponge filters on the intake of the canister, just squeezing them out on a regular basis. This works IF one is careful about not letting the sponge fill up with debris and stop flowing. The frequency of this cleaning depends on the fish load and food load. If you only have light loading (a few small fish) a sponge pre-filter can handle it. If you have heavy loading of fish (many larger fish)  it won’t handle the load. It also depends on feeding. If you feed 1% of the fish’s weight you might be fine. If you feed 3% of a fish’s weight you could be in trouble.

Another idea which came from Cory of Aquarium Co-op is to put the suction end of a canister on the top of the undergravel filter tube.  It is the equivalent of putting a powerhead on an under-gravel while it does excellent “pre-filter” mechanical filtration for the canister. It is especially good in a tank with fry as the flow into the undergravel isn’t large enough to trap the fry or suck them into the canister pump blades.

But this undergravel pre-canister filter has one huge problem. If the canister springs a leak, ALL the water (and I mean ALL the water) ends up on the floor and you have a tank full of dead fish. So I can’t recommend this method of pre-filtration.

Maylandia estherae Red Zebra Cichlid
Maylandia estherae Red Zebra Cichlid

Other Topics of Interest with Canisters and Filter media

Note that the filter media such as Biohome and De*Nitrate (smaller Matrix) which supposedly remove nitrates by denitrification (reduction to nitrogen gas) simply don’t work. That is delved into with testing in this article:

7.5. Denitrifying Media

For a more in-depth analysis of canisters click on these two links:

8.3.1. Canisters in Depth

8.3.2. Do-it-yourself Canisters

8.3.3. FX Series Canister Filters

Startpage Aquariumscience

Source: – David Bogert

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