7.2.3. K1 Media

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 K1	Static 30 ppi foam
Effective surface area in filter	540 sq.ft. per cubic foot of filter	340 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
Noise	Loud	Quiet
Aeration	Great	None
Maintenance	None	Clean 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.

Our Data

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

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.

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.

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.

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.

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.

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.

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

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.

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.

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/m³	m²/m³ from math
Fluidized K1 media	not tested	600	na	1980	na
30 PPI foam	17	340	400	1122	1320
Pot scrubbers	14	280	80	924	264
Static K1 media	13	260	200	858	660
20 PPI foam	not tested	220	180	726	594
Aquarium gravel	6	120	120	396	396
Blue Matala pads	5	100	120	330	396
Eshoppe bioballs	5	100	60	330	198
¼ to ½ inch lava rocks	3	60	60	198	198
Matrix	3	60	30	198	99
Biohome ultimate	2	40	30	132	99
Ceramic rings	2	40	40	132	132
* 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/m³	ft³ to get 5ft²	m³ to get 0.46m²	ft³ to get 100ft²	m³ to get 9.29m²
Fluidized K1 media (60% loading in sump)	540	1782	16	0.45	320	9.06
30 PPI foam in canister or sump	340	1122	25	0.71	500	14.16
30 PPI foam powerhead operated sponge	340	1122	25	0.71	500	14.16
30 PPI foam air operated sponge	300	990	29		580
Plastic pot scrubbers	280	924	31		620
Static K1 media	260	858	33		660
20 PPI foam in canister or sump	260	858	33		660
20 PPI foam powerhead operated sponge	260	858	33		660
20 PPI foam air operated sponge	230	759	38		751
Powerhead operated undergravel filter	140	462	62		1240
Aquarium gravel in canister or sump	140	462	62		1240
Air operated undergravel filter	120	396	72		1440
Blue Matala pads	120	396	72		1440
1/8 inch garden pumice or perlite	100	330	86		1720
Bio balls	100	330	86		1720
Alfagrog	80	264	108		2160
1/2 inch lava rock	60	198	144		2880
Seachem Matrix	60	198	144		2880
Biohome Ultimate	40	132	216		4320
Ceramic rings	40	132	216		4320
Ceramic balls	30	99	288	8.16	5760	163.11
Expanded clay pebbles	30	99	288		5760
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.

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.

Property	Matrix	K1 type extruded plastic media
Cost per ft3 in bulk	$ 200	$ 50
Surface area in ft2/ft3	30	200
Cost per 100 ft2 surface area	$ 666	$ 25
Does it shed abrasive particles	Yes	No
Is it used by most hobbyists to fill their canisters	Yes	No

Comparison of Matrix and K1 Media

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.

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%”.

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)

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.

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.

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.

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