Sponges as live rock replacement

Does anyone use sponges in their sumps/filters for biological nitrate reduction? By sponges I mean the things you clean your car with, rather than the animals.
I was wondering whether they would work as a kind of live rock replacement in a sump. Do you think the large car washing sponges would be large enough and dense enough to generate an anoxic interior for nitrate reduction? Or would they not be dense enough to slow the diffusion of oxygen through them enough to create an anoxic environment?
What do you think?

 

it doesnt seem it would, filters are made out of much trial and error so the way they are made now with refugiums and such is the best way to use them, a sponge would be a nitrate factory just use some spare base rock or some ceramic rings.

 

On top of that, many sponges have antimicrobial agents on them. Those would kill any biological filtration in your tank and cause a crash.

 

Sponge will help do two things: provide a surface for nitrifying bacteria and trap detritus.

The surface area a sponge provides doesn't even compare to the extremely porous surface area of live rock... so the amount of bacteria that can colonize a piece of sponge to an equally sized piece of live rock is far less. This isn't a problem for the sponge, but it just means that the sponge is less efficient for its size in converting ammonia to nitrite and later to nitrate.

One problem with sponges is the large pores allow for detritus to get trapped, build up and decay. Rock's pores are too small for this to become too much of a problem.

Lastly, rock can have anoxic, or oxygen depleted areas, which allows for specialized bacteria that further the reduction of nitrates to N2. Sponges obviously don't offer this.

If you have a sponge that is removed and cleaned every day, then the conversion of ammonia to nitrates will take place, and the trapped detritus is removed... this can be beneficial, but requires a lot of maintenance.

 

^ yup

 

I thought as much.
Damn it, dry rock is so expensive! $15NZD ($12USD) per kg here. When you want 50kg, it becomes ridiculous!
Thanks tho.

 

why do you want rock in your sump?

if you have enough in you tank, you dont need any in the sump

sumps are ideal as an extra volume of water
great places to keep equipment so the main tank is not cluttered
but unless you have a large predator type aquarium, with minimal rockwork to maximise swimming room for the inhabitants, then IME , you dont need any extra biological filtration capacity in the sump

if you want some form of mechanical filtration to remove detritus (floaties that make the water look cloudy) then a filter sock at the end of the overflow into the sump
or passing that water through filter padding or a sponge that you clean weekly, would work IME

Steve

 

Thanks Steve.
My thinking was that I could use a live rock substitute in the sump and use something less expensive (but not useful in bio-filtration) in the main tank. Eg a PVC frame that I can put a small amount of rock over to give the impression of a large rock pile. That way, the filtration work of the live rock is carried out by things in the sump and the tank looks good without spending $1000s on rock.
I guess my thinking was a little too ambitious!
I might try making some base rock from cement. As I mentioned, dry base rock here is $15/kg at the cheapest LFS, and I would like to avoid paying that as much as possible. I have enough in my tank for now, but would like to do some aquascaping and require more rock.