Rüdiger Rautenberg on a cheap filtration method deserving of greater exposure…

The HMF (Hamburger Matten Filter) is apparently not all that well known outside mainland Europe, or so I’ve been told. I cannot speak for the whole of Europe but in Germany it is certainly a widespread and well-loved method of filtering fish tanks and is favored especially by breeders and hobbyists who maintain a larger number of tanks in a dedicated fish room.

This article will only briefly touch on exactly how the filter works since that is a rather complex subject and has a lot to do with the very basics of filtration in a fish tank. There could be a separate article on that if enough people are interested. Here we concentrate on the general set-up of the HMF, its main advantages and how to build one.

The history of the HMF is somewhat shrouded. Its origins are assumed to lie in the former DDR (East Germany) but there’s no confirmation of that as yet. And strictly speaking, the HMF is not really an invention but rather a further development and improvement of the well-known sponge or bubble filter.

Regular sponge filters have a distinct disadvantage in that they are only suitable for smaller tanks. If you wanted to run them in anything larger than a standard 54 l, you’d need a whole array of them, which takes up space and is rather unsightly.

The medium of the HMF is composed of sponge sheets (‘Matten’), which should be available at either your LFS or a specialist shop for fish ponds. I have seen them in the pond section of the larger hardware stores too. The size of the pre-cut sheets usually starts at 50 x 50 cm and can go up to 100 x 200 cm.

Different grades are available but for our purpose only two are really relevant. I recommend 30 ppi (pores per inch) for the general show tank and 40 ppi for a breeding tank. Anything below 30 ppi loses its mechanical filtration properties to a certain degree, resulting in suspended solids in the water column. Anything above 40 ppi clogs a lot faster and thus requires more maintenance.

It comes in 10, 20, 30, 40 and 50 mm thickness of which I exclusively use the 50 mm variant. Additionally the sponge comes in two colors, black and blue, of which I personally prefer the black since it is less conspicuous in the tank (unless of course you have a blue background). But it is (for some unknown reason) more expensive than the blue variety.

The HMF started out as a simple separation of the tank (front to back and top to bottom) by means of a sponge sheet, driven by an uplift tube plus diaphragm pump. The space behind the sponge needs to be just big enough to accommodate either the uplift tube or  motorised pump depending on your preferences.

I personally use motor pumps simply because I just cannot stand the noise of a diaphragm pump, which is needed to operate the uplift. Additionally, with the motorised pump, I know the exact capacity, i.e., how much water is moved per hour. So all four examples introduced here are based on motorised pumps as a means to transport the water.

The water is pumped from the small compartment behind the sponge back into the main part of the tank. From there the water has to pass through the sponge into the pump compartment, i.e., the “filter chamber”.

The mechanical filter capacity of the sponge has already been mentioned but much more importantly, it is chemically inert and offers a huge surface area for beneficial bacteria to colonise and hence it is chiefly a biological filter (suggested further reading for those unfamiliar with biological filtration: search the web for “Nitrogen cycle” or read the article “Cycling an Aquarium” on these pages.)

This basic design has two tiny disadvantages I don’t like, however.

1. when the foam starts clogging with usage (which is inevitable), it will bend towards the suction of the pump and allow unfiltered water to flow past the edges.
2. the filter surface area the water flows through is a given due to the tank dimensions, and one has to adjust the capacity of the pump to allow for effective biological filtration (see formula later in this article).

For these reasons I prefer the “HMEF” (Hamburger Matten Eck Filter), which is nothing but a “corner variant” of the HMF. Due to the arch of the sponge it cannot be sucked inwards (given the right thickness) thus it holds its shape.

To set this up I use two lengths of angled PVC, siliconing these to the glass (please be sure to ONLY use aquarium safe silicone) and add a strip of polypropylene (PP) or similar at the bottom. This strip serves to prevent gravel or other substrate from entering the “filter chamber” when I need to remove the sponge for cleaning:

Then I slide in the sponge:

In this case I decided to position the filter outlet at about the 1/3 of tank height. This is not important for the efficacy of the filter, only in terms of the desired water flow in the tank:

Because the water level was intended to be at the top of the tank, I closed the opening with a piece of sponge to prevent fish from entering. Since it was an open tank I added some hydrocultured plants to make the whole thing a bit prettier (the heater was later moved into the “filter chamber” thus becoming invisible):

The frontal shot of the tank was taken less than 30 minutes after setting it up. As you see, the filter has already cleared the water almost entirely. But what you see too is that the filter isn’t an eyesore at all. It will be even less so once the plants have grown a bit:

So, how big must the filter be, and how do we decide the dimensions?

That depends on a number of factors including:

1. Water turnover per hour
2. Flow-rate
3. Volume of the tank

To achieve effective biological filtration the following parameters have proven themselves as most efficient.

– The water turnover should be between 2 and 3 times the tank volume per hour.
– The flow-rate should be between 5 and 10 cm per minute.

The maths

For our example let’s take a standard 160 l tank (100 x 40 x 40 cm) and as the golden medium a turnover of 2.5 x volume/hour and flow-rate of 7.5 cm/minute. We have a 160 l tank and 400 l/h pump, but how do we achieve the correct flow-rate?

Flow-rate refers to how much water flows through a filter sponge of given surface area (taken as a cross-section, i.e., sponge depth x sponge height) within a certain amount of time. There is of course a formula for this:

Cross-section [A] = Volume [Q] x Water turnover [n] x 1000 / (Flow rate [V] x 60)

in our example:

A = 160 x 2.5 x 1000 / 7.5 x 60 = 400 000 / 450 = 888.88 cm² ≈ 890 cm²

Since the height of the tank is a fixed we simply divide this result by 40 (tank height in cm) to calculate the width of the piece of sponge we need, i.e., 890 cm² / 40 cm = 22.25 cm, ≈ 22.5 cm. Therefore we cut out a piece of sponge measuring 40 x 22.5 cm.

The only thing left now is to calculate where we have to silicone the PVC angle to the glass to achieve a nicely-shaped  filter which is basically a quarter of a circle.

The circumference of a circle = π (pi) x diameter or π x 2 x radius. In this case we can easily calculate the circumference by multiplying the width of our filter material by four (4 x 22.25 = 89 cm) but now require the radius as this corresponds to how far away from the tank corner we should position our PVC angles. This is calculated as follows:

r = c / 2 x π = 89 / 2 x π = 14.167899, ≈ 14.5 cm.

In the tank we now measure 14.5 cm from each side of the corner where the filter is supposed to go and mark where to place the PVC angles at both the top and bottom. The rest is a piece of cake.

In summary…

I hope I could bring across how easy it is to build a really effective, low cost and low maintenance filter. The advantages of the HMF can be quickly summarised:

• Its biological efficacy is rivaled only by more expensive, much larger and more difficult to build filters such as fluidised beds or wet and dry trickle filters.
• It is dirt cheap to build and operate – you don’t have to buy new filter material for years after.
• Very low maintenance – the one pictured above ran for more than a year with no maintenance at all before I had to clean it.
• When it becomes necessary to clean it, it takes about 15 minutes if that.
• Additional equipment, e.g., heater, CO2 diffuser, etc., can be placed (hidden) in the “filter chamber”.

After some years of experience with the HMF, it was, is and will stay my first choice of filtration for freshwater as well as brackish aquaria.

To round off this article, I’d like to show a few more examples of HMFs that I build for different purposes.

First the one I built for my 160 cm long “Asian river”. You can see that the basic set-up is the same as in the one above except for a hole at the bottom.

As an outlet I built a modified “river manifold”:

Again you can see that the filter fits into the tank quite nicely without being too conspicuous. The picture was taken about 15 minutes after the tank was completely filled.

Next is a “mobile” HMF fitted with a spraybar I built for a 54 l dwarf puffer tank:

And finally another mobile solution that was designed for my ≈ 30 l special-purpose “buckets”:

N.B. The outflows in all 4 models cannot be driven by uplift tubes since the latter do not produce sufficient water pressure, and the outlet should be just above the water surface (yet another reason I prefer the motor pump).

Filter foam is best cut with a run off the mill Stanley knife (broad blade) with new blades. Use a guide, like an aluminium angle or ruler, but don’t press down, just gently hold it in place so it can’t move. Don’t exert much pressure on the knife, just gently slide it through the material and cut in 2 or 3 passes.

For those who understand a bit of the German language I recommend the following web site for further reading: www.deters-ing.de

Olaf Deters is one of the guys who spent a lot of energy improving on the original HMF design. He’s a well known aquarist, specialising in killifish, and you can read his article in the German-language version of the magazine Amazonas number 44 Nov / Dec 2012 pp. 28 – 39.

Concluding I would like to emphasise that I by no means have “invented” nor have been involved in the “invention” of the HMF or any part thereof. I have though, where necessary, modified it to my and, more importantly, my fishes’ needs.