You don't want to hear "scrubbies" which are dirt cheap and have a high suface so, here's a comparative list for you to choose for yourself.
Bio Filtration takes place in all the nooks and crannies of your pond, including your plumbing pipes, liner and tubing. The brown slimy deposits that accumulate in your pond plumbing are colonies of beneficial bacteria or Bio Film Colonies , creating what is known as the nitrogen cycle. For years, "lava rock" has been used as a bio filter media with good results. There are however, much more efficient, tested and proven media on the market today which will do the same job in much smaller containers. Here in So FL Lava Rock are colored clinkers ( the by product of burning Coal) The clinkers, while being porous, will clog unless you have a very good pre filter system. They do not allow water to flow through them, which wastes valuable filter space. All the little nooks and crannies in the faux Lava Rock eventually become clogged with algae, mulm, sand etc. making the effectiveness the same as throwing pieces of granite into your bio filter. With most homemade bio filters there is a certain redundancy built in (bigger is better) so it will take some time depending on your stocking levels and filter system, for the bio filter to lose it's effectiveness.
Lava rock is inexpensive to replace but, over the long run it is a maintenance nightmare. The sheer weight, abrasiveness and time spent cleaning, should be enough to steer you away from this product. We've cleaned many a lava rock filter and I can tell you, that trying to clean this media with anything less than a steam power washer, is an exercise in futility and will not get it back to it's original surface area.
Surface Area and Packing Density
While surface area measurements will give you some information, the way the media packs into a container, should also be taken into consideration. Strapping tape, for instance, is impossible to pack efficiently, whereas nylon pot scrubbies will pack much better into a container resulting in more surface area with less space.
DIY
For the do-it-yourselfer, some bio media to consider in order of effectiveness and cost:
Nylon Pot Scrubbies (see picture), Plastic Strapping Tape, Springflow media and PVC Shavings.
I purposely omitted the bio-balls, bio squares and baked ceramic media, Matala Fiber and Kaldness media because of their high cost. The effectiveness of a bio filter is controlled by the surface area of the bio-media, bio bugs need a constant flow of oxygenated water, to be able to eat, breathe and reproduce. More surface area provides more habitat, therefore, you can substantially increase your filter surface area and lower the space needed, by using the right media.
A quick Comparison
To calculate the surface area of any bio media , dip it into a measuring tube to find its volume. Do the same with the other media and compare. More volume of solid means less surface area. More volume is just taking up valuable space.
Cleaning
I recommend cleaning the bio media only if you can see a 1/4 " build up of mulm, and then cleaning it with pond water, as chlorinated water will kill most of the bacteria and cold water will retard their growth. You should keep the filter material moist while servicing your filter, as drying will kill most of the bacteria.
If you like Math
Japanese mats have a specific surface area of about 275 m2/m3. So one mat of 2x1x0.04m is about 80 liters and has a total surface area of 22m2.
Depending on a lot of factors bacteria are able to convert between 0.2 and 1 gram ammonia per m2 surface area per day. When you calculate with 0.2 you are pretty safe.
We already calculated that the jap mat had a surface of 22 m2, so one mat is able to convert about 4.4 gram ammonia per day.
About 2-4% of the amount of food given per day is coming back as ammonia in the pond water, depending on protein level among other things. Calculating with 4% gives a good safe margin.
So one Japanese mat was able to convert 4.4 grams ammonia per day. This 4.4 grams is 4% of the amount of food given. So total food given per day can be 110 grams per one Japanese mat.
ed. Comment---I've been using the nylon pot scrubbies for close to 30 years now. When I first started building filters, I used everything I could think of, including cut pieces of well pipe, hair curlers, plastic shotgun shell wads, strapping tape etc. Every other media worked to some degree but after much trial and error, I found nylon pot scrubbies to have certain advantages:
1. They are produced for food contact, and therefore, you can be pretty sure, no harmful chemicals are used in their production.
2.After Laboratory testing they were shown to have more surface area than most media, except for some of the ceramic media i.e Siporax tm, or activated carbon. The sintered medias and A/C, however, have a clogging problem with DOC's creating a bio-film and clogging the pores, which then must be cleaned manually.
3. They do not allow, unlike strapping tape or bird netting, by virtue of their construction, large gaps with no bio filtration, or compacted areas trapping dirt and debris. You get maximum, even, coverage in the smallest container possible.
4. They last for a long time--I still have 12 Year old systems running with the original nylon pot scrubbies, they lost most of their color, but look like they'll last a lot longer yet.
Further Reference by Dr.Tim Hovanec
