7.2.3. K1 Media

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Author : David Bogert

Published :

The translation of this article has not yet begun. There are still open tasks such as converting units or creating tables.

K1 is a chopped plastic media that is used both in something called a “fluidized bed sump” (or a “moving bed bioreactor” or MBBR) and used as a static (i.e. “stationary and unmoving”) media in other aquarium filters. The ability of fluidized K1 to biofilter the water and give crystal clear, very healthy water is simply the best of any media. K1 is the third best static biomedia to put in any aquarium filter, behind foam and only slightly behind pot scrubbers.

What is often missed is how cheap a fluidized bed can be IF AND ONLY IF the K1 can be purchased in bulk. On the basis of pounds of fish that can be well biofiltered, fluidized K1 is only 22% the cost of 30 ppi Poret foam. Indeed, even the stationary K1 is 46% the cost of 30 ppi foam on a per pound of fish biofiltered basis.

The static 30 ppi foam compares to the fluidized K1 as follows:

Fluidized K1Static 30 ppi foam
Effective surface area in filter540 sq.ft. per cubic foot of filter340 sq.ft. per cubic foot of filter
Cost per cubic foot of filter$35×60%=$21$63
Cost per 100 square foot in filter$21/5.4=$4$63/3,4=$19
MaintenanceNoneClean every 3 months
Comparison of Foam and K1 Media

It is important to note that the K1 efficiency is per cubic foot of filter, NOT per cubic foot of K1 filter media. Big difference!

If one is a nerd like the author one might be interested in this long boring article on why K1 is such a good media. It includes testing which proves this is true.

Placidochromis sp. Phenochilus Tanzania Star Sapphire
Placidochromis sp. Phenochilus – Tanzania Star Sapphire

Our Data

The data in the master table for K1 type extruded plastic media is:

Calculations for the surface area of K1 Filter Media
Calculations for the surface area of K1 Filter Media

The surface area of the K1 should be very, very roughly about midway between the surface area of 30 ppi foam and the surface area of the 1/8 inch gravel.

Tropheus duboisi maswa
Tropheus duboisi maswa

There are small extruded and chopped plastic media which look like plastic slices of okra. The original design was the Kaldnes K1 design from Europe. This media has become known simply as “K1” media, even though there are now many different designs and copycats. Helix Floating Bio Media is virtually identical to K1 media. I simply work out the cost per cubic foot of media and buy the cheapest one I can find on the internet at bulk pricing. I buy a LOT of this at one time.

K1 and similar rip-offs are the only media that should be used in fluidized bed filters in home aquariums.

K1 type media
K1 type media

One gallon of media in bulk can be had for $11.49, which isn’t bad. Note that in aquarium media if you are buying K1, ceramic rings, or bioballs you must buy in bulk of at least one gallon. In smaller quantities the price is simply ridiculous. For instance, one liter of K1 is $21, which equates to $76 per gallon. With COVID the price of this media has more than doubled. But the price discounts for buying in quantity are still huge. It is now up to $13 to $25 if you buy one gallon, $55 if you buy one cubic foot (that is $7.33 per gallon). On EBay you can buy 2 cubic feet for $70. That is $4.67 per gallon, $1.24 per liter, or $35 per cubic foot when you buy in bulk of two cubic feet.

But the price is HUGELY variable at the current time. Some suppliers are selling at $13 per gallon, some are selling for $30 per gallon in quantities of a gallon. There is no way to predict where this price will go or even if you can get this media anymore (“supply chain problems”?). And it appears you cannot even get small quantities anymore, at ANY price. Also note that the price also varies a huge amount by country. In some countries it is very cheap and in others very expensive.

Symphysodon aequifasciatus White Discus
Symphysodon aequifasciatus – White Discus

Note there are versions of “K1” called “micro”, “K2” and “K+”. These have smaller openings than straight K1. “Micro”, “K2” and “K+” have more surface area that straight K1. But they are shorter in channel length. And there are a large number of designs that can be found on the web. I have several different varieties of K1 rip-offs in my many K1 fluidized beds (I just buy the cheapest per cubic foot I can find when I need some) and they all seem to work very well.

One of the readers of this website used micro in one canister and regular K1 in another (i.e. a static media). The micro did fill completely with biofloc in four months, which would considerably reduce its effectiveness. The regular K1 was still open. But this is for a static media. In a fluidized bed I have never had a K1 type media plug up, and I got a large assortment of types.

K1 micro filter media
K1 micro filter media

I’ve had K1 rip-off media go for three years in rapidly moving moving media beds with no cleaning and there is still daylight through each of the openings in the media. I.e. the media never “clogs”. Since clogging will reduce the efficiency of the K1 this is important.

OASE Hel-X 13 BioMedia
OASE Hel-X 13 BioMedia

There is a similar product to K1 called “OASE Hel-X 13 BioMedia”. The OASE media looks like it will be equal to K1 media. I am a little concerned the OASE media openings inside are a little small, which means they may well plug in a few months. But I’ve never tested it so I can’t be sure.

OB Peacock
OB Peacock

An exchange with one of the obsessive compulsive readers of this website went like this:


Dave, I bought a bunch of what I thought was K1 media on Amazon. I made several purchases from several different places. I bought the 5-spoked version because I figured that must have more surface area. Now I have figured out that the 5-spoked version is smaller (is it “K2?”). All the 5-spoked versions I bought were the smaller size, even though they were labeled “K1” and sometimes claimed measurements the same as K1. I only figured it out when I bought some 4-spoked K1 and noticed the size difference.

Your K1 article has a picture of the 5-spoked version. The description from the manufacturer says “better than Kaldness K1.” I think that is actually one of the versions I bought, which turned out to be smaller than K1.

The sizes I could find represented on the internet were:

  • the 4-spoked K1 is 12mm in diameter and 9mm high
  • the 5-spoked (K2?) is 10mm in diameter and 7mm high [sometimes inaccurately described as 12mm x 9mm]
  • the Micro is 7mm in diameter and 9mm high, according to evolutionaqua.com (where they sell Micro)

The 5-spoked media that I bought seems to be about the same diameter as the actual K1, but considerably shorter. What do you think of the 5-spoked media as compared to the 4-spoked K1 (12mm x 9mmH)? It would seem that the extra spoke would make it more likely to clog, but the shorter size would make it less likely to clog.


In reply to John … I don’t think there will be any noticeable difference. But I can confirm that the small openings do clog but ONLY in a static mode. There is no data or journal articles on it. I have done no formal testing. So I simply do not know for sure. But my anecdotal evidence is that ALL K1 type media performs the same in a fluidized bed.

Maylandia zebra Blue
Maylandia zebra Blue

Static K1 Media

This K1 plastic media has a large surface area which is easily accessible to both bacterial growth and high water flows. This is a great combination for fluidized beds. What everyone is missing is that they are an excellent media for use in any type of submerged static media bed filter. They have one of the best combinations of price, ease of use and biofiltration area of any submerged static media.

The label for the K1 media quite correctly says:

  • Large protected surface area 950 m2/m3 (290 ft2/ft3)
  • Can be used in Static or Moving Bed Filter Systems
  • Ideal for use in External Aquarium Filters

ALL the points on this label are 100% correct. This is very refreshing and is one of the few times I’ve seen 100% honesty from a supplier of aquarium products. The most important point here is:

Static K1 is a great media for canisters and no one knows it!

K1 can be put in the middle trays of all canisters. In the top tray of a bottom to top flow canister the K1 needs to be in a bag to keep it from floating out of the canister.

Metriaclima tarakiki zebra slims
Metriaclima tarakiki zebra slims

Testing Static K1

A test was run on all the various filter media in actual aquarium conditions. By this test K1 was exceeded only by foam and plastic pot scrubbers. It was five times better at ammonia oxidation that lava rock or ceramic rings.

A test of ammonia oxidizing capability of various filter media was run. The first number, the “efficiency” is the average ammonia oxidizing that 15 cubic inches (245,81 cm³) of media accomplished over a 90-day period. The second number is the “effective” surface area in square feet per cubic feet calculated from that test. The third number is the effective surface area in square feet per cubic feet calculated by simple mathematics. The correlation between the test results and the calculated surface area is very significant and means the testing was accurate.

Media“Efficiency” from two tests *“Effective” surface area ft²/ft³ ft²/ft³ from math“Effective” surface area m²/m³m²/m³ from math
Fluidized K1 medianot tested600na1980na
30 PPI foam1734040011221320
Pot scrubbers1428080924264
Static K1 media13260200858660
20 PPI foamnot tested220180726594
Aquarium gravel6120120396396
Blue Matala pads5100120330396
Eshoppe bioballs510060330198
¼ to ½ inch lava rocks36060198198
Biohome ultimate2403013299
Ceramic rings24040132132
* average ammonia oxidizing that 15 cubic inches (245,81 cm³) of media accomplished over a 90-day period

A series of extrapolations (i.e. “educated guesses”) was then made.

Biomedia“Effective” surface area ft²/ft³“Effective” surface area m²/m³ft³ to get 5ft²m³ to get 0.46m²ft³ to get 100ft²m³ to get 9.29m²
Fluidized K1 media (60% loading in sump)5401782160.453209.06
30 PPI foam in canister or sump3401122250.7150014.16
30 PPI foam powerhead operated sponge3401122250.7150014.16
30 PPI foam air operated sponge30099029580
Plastic pot scrubbers28092431620
Static K1 media26085833660
20 PPI foam in canister or sump26085833660
20 PPI foam powerhead operated sponge26085833660
20 PPI foam air operated sponge23075938751
Powerhead operated undergravel filter140462621240
Aquarium gravel in canister or sump140462621240
Air operated undergravel filter120396721440
Blue Matala pads120396721440
1/8 inch garden pumice or perlite100330861720
Bio balls100330861720
1/2 inch lava rock601981442880
Seachem Matrix601981442880
Biohome Ultimate401322164320
Ceramic rings401322164320
Ceramic balls30992888.165760163.11
Expanded clay pebbles30992885760
Cubic inches to get 5ft² = (5/E.A.)x1,728 100ft²=(100/E.A.x1,728)

The higher the numbers here the better the media. Fluidized K1 is hands down the best media. Once again, it is important to note that the K1 efficiency is per cubic foot of filter, NOT per cubic foot of K1 filter media. It only takes 9.6 (16x 60%) cubic inches of actual K1 media to get five square feet of effective surface area in a fluidized bed.

Note people tend to obsess over these numbers and try to determine by exacting math exactly what will give them exactly 100 square feet per pound of fish and thus crystal clear water. This exactitude is not warranted. There are at least ten variables affecting this (aeration, water movement, feeding, pH, KH, food protein level, etc.) and they can react in very unpredictable ways. So these numbers are only VERY gross approximations.

Tomocichla tuba Tuba Cichlid
Tomocichla tuba Tuba Cichlid

How Does Static K1 Stack Up?

A comparison is in order. This table compares K1 extruded plastic media to Matrix media as static biomedia in a canister filter or sump filter.

Comparison of Matrix and K1 Media
Comparison of Matrix and K1 Media

What this can translate to is illustrated by the number of fish a typical canister can handle stocked with these two media:

• Static K1 media – 20 five-inch fish

• Matrix – 3 five-inch fish

So Matrix costs four times more than K1 on a volume basis and a given volume of Matrix only can handle one sixth the load of a given volume of K1. This is a difference in cost effectiveness of a factor of 24, which is huge.

OB Peacock
OB Peacock

K1 in Fluidized Beds

The biggest use of K1 is in fluidized bed filters in sumps. The K1 media is loaded into the sump at about 60% of the available volume. Then the K1 is “fluidized” and kept in constant motion by the bubbles from air stones or wavemakers. Most advanced aquarists including the author are now using this type of filter quite successfully for any aquarium over 75 gallons (250 liter).

This fluidized bed is simply the best filter that there is. In a fluidized bed filter the K1 has an effective surface area of roughly 540 square feet per cubic feet of filter volume. This is roughly twice the capacity of both 30 ppi foam and static K1 media.

Note there are some who say only to load the K1 media at 25% loading by volume of the sump. The 25% is based on the myth that it is the flow rate which determines the efficiency of the media. If you load 25% ALL the media and really pour the air to it, it is ALL constantly in movement. If you load 60% then about 25% is constantly in movement and some 35% of volume is stationary with water flowing through it.

This second scenario will give a much greater surface area for beneficial bacteria and will give much better filtration. Only the water needs to move for K1 to be effective. The seminal work on K1 media (“Advanced Compact Wastewater Treatment Based on Coagulation and Moving Bed Biofilm Processes”, Odegaard, 2000) says the “standard filling is 67%”.

Protomelas taeniolatius OB Red Empress
Protomelas taeniolatius OB – Red Empress

Other Fluidized Bed Media – Mutag Media

There is a type of media out of Germany called the “Mutag Biochip” which is a polyethylene foam disc. It probably (I haven’t tested it) will work about as well as K1 in a fluidized bed but it is somewhat difficult to keep in motion without it stacking up and stopping biofiltration. It can be made to work very well in commercial operation where there is constant engineering support. We do not feel that it is good for an aquarium fluidized bed. Note it cannot work in a static bed because of the tendency of the disks to stack with one another, like the chips in a box of Pringles.

Note also that, like most commercial media, the surface area claims for this product are just plain old marketing hype. They say they have 5500 m2/m3 (1,679 ft2/ft3)

Mutag Biochip
Mutag Biochip

Fluidized bed Media – Foam Products

There are several types of foam media which are made both for fluidized beds and for static filter beds. They are typically one inch cubes or “X”s of about 30 ppi foam. Poret®aqua Cubes are one brand.

Poret aqua Cubes
Poret aqua Cubes

These typically come from Germany. They simply won’t work for a variety of reasons.

First off there is no flow though the urethane foam when it is in the water. The pores in this foam are too small. They will resist the flow of water through them far too much to be effective. Water will always take the path of least resistance.

The second not so obvious reason is than urethane has a specific gravity of 1.2. This means that when these foam pieces become waterlogged after a few weeks of operation they will simply fall to the bottom of the tank and stay there in a sodden useless mass. The material K1 is made of, polyethylene, has a specific gravity of 0.95, which means it floats forever and thus performs quite well as a fluidized bed media.

A reader has used both the Poret Aqua-media foam cubes and the K1 media in fluidized bed sumps. The cubes tended to not circulate, absorbing air and floating if the sump is air driven and absorbing water and sinking if the sump is powerhead driven. They also did not perform near as well as the K1 media as far as biofiltration, with a noticeable degradation in water clarity with the cubes. This was completely predictable.

Otopharynix lithobates
Otopharynix lithobates

Fluidized Bed Media – Plastic Straws

One little oddity some DIY websites push is ¼ inch cut up sections of plastic straws. This actually is a very good media if one can find a cheap source of plastic straws (NOT Amazon!). I found a package of plastic straws for $1.00 at my local supermarket in the cheap goods section. It works out to 40 cubic inches. This in turn works out to $43 per cubic foot and about $19 per pound of fish filtered.

So if one has the patience to do a lot of cutting (a paper cutter would be useful), one can make a straw fluidized bed media. Just make sure they have a cut length about the same as the diameter of the straw. This homemade media should be very close to K1 in performance. Note I haven’t done any testing so that is an unsupported opinion only.

Trichromis salvini Rio Candelaria
Trichromis salvini – Rio Candelaria

Also note that the whole straws can be used to make some very interesting filtration schemes. For instance, one can pack the baskets in a canister with straws oriented in a vertical position. The flow would be very good and the straws would have a very good surface area. Making and testing such an arrangement is on my “to do” list.

Note that most canisters must have the baskets in the canister in order to operate. The baskets typically have a pipe as part of their construction which shunts the water. This pipe formed by the baskets is integral to the operation of most canisters.

Alternatively a length of 2 inch (5 cm) PVC pipe can be packed with whole straws. The water could be fed to the filter pipe via a tube from a small 3 watt fountain pump. The flow would be down the length of the pipe (and the length of the straws). This would appear to be a very good arrangement for a small nano planted tank. We show several such designs in this link:

8.7.8. Small DIY Filters