While I was browsing overseas aquarium web sites and forums last year, I came across a couple of terms I was not
familiar with - “Hamburg-Matten filters” and “Poret”. It turns out that the Hamburg-Matten is a type of filter in common use
in Europe for the past twenty years and that Poret is the brand name of the “special” foam used to make the best of them.
From what I could work out, the Hamburg-Matten filter could be as simple as a block of Poret (or similar) filter foam
set up so that the aquarium water was continuously passing through it. The “secret” (if there is one) is to maximise the
surface area of the foam acting as both a mechanical and biological filter and to ensure that the flow-rate was optimised
for the growth of the “good” bacteria that do biological filtration (converting ammonia to nitrites and nitrites to nitrates).
Poret foam is a semi-rigid open-cell polyether foam (it is very similar to the foam supplied with Eheim cannister filters)
and comes in several grades from coarse (at 10 pores per inch) to very fine (45 pores per inch). It is most commonly sold
in blocks that are 5cm thick but 3cm and 7.5cm thickness are available in some densities. The foam is very high quality
and can last many years without the cells degrading or collapsing as happens with some cheaper (and even some more
expensive!) alternatives.
Photo 1: The open cell structure of Poret filter foam provides a
massive surface area for bacteria and allows the free flow of water.
This picture shows 20 Pores Per Inch (PPI) medium density foam
(left) and 30 PPI high density (right).
In its simplest form, A Hamburg-Matten filter system looks like Figure 1.
It really is very simple, both in theory and practice. The powerhead or airlift moves water to the main area of the tank
and it flows back through the foam to the narrow reservoir. The only “trick” seems to be ensuring that the flow rate is
neither too low nor too high to get the optimum biological filtration through bacterial nitrification. You can do a lot of maths
and science to figure out the “right” flow rate (there are papers on the internet about it, of course) but there is also an
easy “rule of thumb” you can use for “normally proportioned” tanks - make sure the flow through the filter mat (the flow
rate of the powerhead or airlift) is between 3 and 5 times the tank volume per hour.
That means, for example, that for a 60cm x 30cm x 30cm aquarium (a standard 2 foot tank) the flow rate should be
between around 150 and 250 litres per hour. There are many small, cheap powerheads in that range. On the same
basis, a 90cm x 45cm x 45cm tank (180 litres) would need a powerhead moving between 540 and 900 litres per hour. Do
note that these flow rates are very low compared to what some of us have regarded as normal or adequate for a good
biological filter. In some applications you might need to provide additional water movement or aeration on the “tank side”
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