1 litre of media per 25l of tank water

[SPACKlick]

Even when they're clean, the pores are too small for any significant water flow throughout and once there's a biofilm of usful bacteria on them you just get the outer surface so you've lost the internal volume and it smooths off some of the fine texture of the surface.

Whereas, because the pores of sponges are open to flow your nitrogen is going through a 3d matrix of biofilm. I'd rate a really fine sponge (30-35ppi) as more than twice as good as sintered glass. And my preferred media is 20ppi foam because you have to clean it less and it's nearly as good as the fine foam.

[black ghost]

They’re not even comparable.
Siporax has an internal surface area of 450m^2; per litre.
30ppi foam has a surface area of about 1 ft² per litre.

If it is kept clean these surface areas are available. Porous media were not designed for additional nitrification, but for denitrification, and they work very well, if they’re kept clean, as proven by many controlled tests over the years.

[Stephen]

I do not use sponges as Biological Media, I use them as Mechanical filtration.

It doesn't matter if you use them as biological media or not, the bacteria do. Aerobic Bacteria grow better in 20-30ppi sponges than they do in sintered glass and ceramics from everything I've seen and from personal experience with small internal filters.

I'm only saying that I use sponges as Mechanical filtration (to get rid of the debris etc..) and have dedicated good quality Biological Media to house the majority of the beneficial bacteria.

As with all Fluval external filters, the mechanical media (sponges) are good but the biological media let them down and need upgrading.

I hard disagree, I've seen someone run an FX6 on 100% foam and heavy load (C. 350L with around a couple of dozen Fancy Goldfish) and, once they added an intake sponge, that filter kept the water pristine clean and the nitrogen cycle fully handled. All the expensive Biomedia; {Substrat, Matrix, BioHome, Alfagrog, Bioballs} are, in my experience, pretty poor as homes for munchers with Ceramic rings being pretty much the worst. I can't find anything in the scientific literature to support any static media as more effective than sponges.

That's what I'm saying.
The Fluval Biological media (ceramic rings) is rubbish.

[Stephen]

The Sera Siporax ceramic rings are a good biological media, probably the best 'ceramic ring' type media, but not as good as other brands such as my Eheim SubstratPro (sintered glass media).

Siporax is not ceramic. It’s sintered glass. It has the same surface area as Substrat pro.

I know that.
Just saying ceramic ring type media, I was talking about the shape.

[SPACKlick]

They’re not even comparable.
Siporax has an internal surface area of 450m^2; per litre.
30ppi foam has a surface area of about 1 ft² per litre.

If it is kept clean these surface areas are available.

Whilst that may be strictly true that there is more surface area (although what I've seen suggests more like 8-12 m²/litre for foam). Every test of it I've seen results of shows the porous media to be innefective internally for supporting any bacterial action. Water gets absorbed through but it doesn't flow, so any bacteria that penetrate end up in stagnant water without access to fresh Nitrogen to consume. To get any reasonable flow through the media you have to constrain the space the water can flow through significantly. And in real world situations there will always be some detritus build up from the biofilm the munchers form blocking flow. As I said, every test and study I've seen puts foam at about twice the biological capacity of sintered media.

Porous media were not designed for additional nitrification, but for denitrification, and they work very well, if they’re kept clean, as proven by many controlled tests over the years.

I've never seen any controlled test showing any denitrification in porous media. The closest ones I've found had plants in the tank that weren't accounted for. I'd be interested in seeing the tests you've read because contacting Seachem and Biohome reveals they don't have any tests at all to indicate it actually works to remove nitrates. And it doesn't make sense that it would work, Denitrification is anaerobic but the nitrates are in oxygen rich water so you'd need some sort of semi permiable barrier letting nitrate through but not oxygen, which is smaller than nitrate.

[black ghost]

Every test of it I've seen results of shows the porous media to be innefective internally for supporting any bacterial action. Water gets absorbed through but it doesn't flow, so any bacteria that penetrate end up in stagnant water without access to fresh Nitrogen to consume. To get any reasonable flow through the media you have to constrain the space the water can flow through significantly.

And it doesn't make sense that it would work, Denitrification is anaerobic but the nitrates are in oxygen rich water so you'd need some sort of semi permiable barrier letting nitrate through but not oxygen, which is smaller than nitrate.

Hmmm. The reasons why you think it doesn’t work are the reasons why it does. Inside the pores the available oxygen gets used up quickly, providing the anaerobic environment required by the anaerobic bacteria, which then take their oxygen from nitrate.
It’s easily testable. In fact, if you make a nitrate filter with porous media you’re pretty much guaranteed to see nitrite, until you’ve tweeked it to the right flow rate.

[SPACKlick]

Hmmm. The reasons why you think it doesn’t work are the reasons why it does. Inside the pores the available oxygen gets used up quickly, providing the anaerobic environment required by the anaerobic bacteria, which then take their oxygen from nitrate.

But Oxygen gets in more places than nitrate. There's no reasonable mechanism for selectively transporting nitrate over oxygen into the porous media.

It’s easily testable. In fact, if you make a nitrate filter with porous media you’re pretty much guaranteed to see nitrite, until you’ve tweeked it to the right flow rate.

It is really easy to test, and no test I've seen has shown any effect of porous media that wasn't seen, and usually seen faster/bigger in foam media. If you've seen these tests, please point me to them. Until I see them I'm reluctant to believe they exist because a) A thorough search of the literature over the last few years has found nothing. b) Commercial waste treatment plants spend a great deal of money on complex denitrifying set ups that would be wasted if a vat of sintered glass could do it. c) The companies that sell this stuff don't even have a basic test that shows it works to point to when asked d) The physical chemistry doesn't make sense.

To elaborate on that last point, anaerobic denitrification requires 1) eliminating oxygen, 2) admitting a steady flow of nitrate, 3) Admitting a steady flow of electron donors which are usually large (relative to the previous two) organic carbohydrates. These three things are incompatible in our cannisters.

[Lo1]

[Lo1]

I've never seen any controlled test showing any denitrification in porous media. The closest ones I've found had plants in the tank that weren't accounted for. I'd be interested in seeing the tests you've read because contacting Seachem and Biohome reveals they don't have any tests at all to indicate it actually works to remove nitrates. And it doesn't make sense that it would work, Denitrification is anaerobic but the nitrates are in oxygen rich water so you'd need some sort of semi permiable barrier letting nitrate through but not oxygen, which is smaller than nitrate.

[Lo1]