Alfagrog and Biofilters for Fish and Duck Ponds

Algfagrog is what we call a pond filter biomedium... bacteria grow on the surface and these bacteria in turn purify the pond water keeping fish healthy in a pond environment. I got this email from one of my newsletter readers and it has a general appeal so I'm publishing it here.
Hi Anthony
As a new subscriber, I have been reading your words of wisdom with great interest – especially those about not succumbing to the temptation of buying overpriced biofilters. I built my own filter 20 years ago (hand packed concrete, 0.8m3 capacity, partially filled with 13mm crushed stone in orange bags packed in such a way that I get largely horizontal water flow, thus keeping the anaerobic bacteria at bay) Not elegant, but it has never let me down. As a biologist turned engineer, I understand how these things work. I do, however, have to lift the stones once a year and vacuum out the sludge with a submersible sump pump.
Which brings me to my first point…….. I am no longer a spring chicken and lifting the crushed stone has become arduous. Substituting them with Alfagrog seems like a good idea to me because a significantly smaller volume would be required. The sludge I refer to is mainly decomposed leaves from an overhanging tree – my leaf trap does not catch them all. (By the way, evergreen is not the opposite of deciduous. All trees lose all their leaves every year – the only difference is that the deciduous ones are considerate enough to drop them all at pretty much the same time) How does Alfagrog behave under these conditions? I imagine that, with thousands of little holes and passages, it would be susceptible to rapidly glogging up and in need of more frequent rinsing. If so, what are the mechanical filtration options? Are brushes any good, or could I use shade cloth curtains inside the biofilter? I do have space for either.
My second point is that I am about to build a duck pond, and this too will be sufficiently close to trees to suffer from the same leaf-drop problem. I can solve it partially with a more sophisticated leaf skimmer but it will not catch them all so the Alfagrog clogging question arises again. I really do not want a swimming pool sand filter anywhere in the system – channeling, lack of space, noisy surface mounted high pressure pump, high electricity consumption compared to Oase submersibles etc. On biological issues, I have assumed that nitrogen production from one 800g duck is equivalent to that of a 3Kg koi and will size my filter and duck population accordingly.
I look forward to hearing from you
Kind regards

Humphrey McAllister

And my reply

Humphrey
Thanks for the email.
Alfagrog has a bulk density of about 0.5 kg per litre. It is highly porous but in my experience so long as occasional back wash (or dunking of the orange bag into a container of pond water) carried out it does not easily block.
To reduce any tendency to blocking brushes or foam or "scrunched up" shade netting as you use now will remove much of larger debris but most of the blockage if any will be from dead algae which most brushes and foams will not necessarily remove.
One thing if you do exchange Alfagrog for stones do not remove all the stone until Alfagrog is established.
Duck ponds ... I have always been wary of suggesting any kind of biofilter for duck ponds because of extreme waste and general mess they create.
You can get Alfagrog from Ken Lombard 012-666-9888 if you want some ... he can also supply liner 500 micron or thicker (as used in large reservoirs) if that's what you want for the duck pond. See http://www.fibrefeatures.co.za

It might be worth having a chat with Ken ... he has a very large koi pond and biofilter filled with Alfagrog which has been in operation for about 3 or 4 years.

Tony

PS Alfagrog is called SUPRA in USA
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