Organic carbon, redox potential and substrates

dkfennell

Junior Poster
May 14, 2005
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I just finished digesting the May Barr Report on organic carbon, and I wanted to see if I understood the part about redox potential. As I understand it, Tom concludes the following:

1. Reduction in redox potential is generally good: first, on setting up of the tank; and second, generally as long as it doesn't go below 0 mV.

2. Reduction of redox potential is a direct result of accumulation of organic carbon, and organic carbon continues to increase in a tank unless physically removed from the tank by the aquarist (for lack of a better term).

3. (I am reading between the lines here:) It is not "compactness" of the substrate that causes anaerobic conditions, but accumulation of organic matter and the microbes it allows the accumulate, because a large biomass of bacteria can reduce the oxygen in the water column sufficient to create anaerobic conditions in the substrate.

4. Once the redox potential goes much below 0 mV, there is no way to prevent the bacteria from creating toxic compounds (hydrogen sulphide or methane), without removing the organic carbon. In other words, aerating the water column, punching holes in substrate to allow increased water flow, mixing substrate, will not help, unless organic matter is removed from the tank.

5. In view of the foregoing, a general substrate overhaul is recommended at least once a year.

Now, I didn't quote chapter and verse, and I sharped up what I consider to be the conclusions, but I did so to see where these conclusions should be modified or softened (if at all). I'd appreciate any reaction.

Next, a specific question. I am not one for fushy aquascaping. In fact, once I plant, I'd just as soon see the plants stay there. In fact, aside from the necessary harvesting of plant matter to allow for a current, and to let fish get around a little bit, I'd just as soon see a lot of plant mass than an Amano or Dutch tank. As a result, my tanks all have carpets that cover the top of the substrate. In fact, if it weren't for the fact that the substrate is seen in cross-section in the front of my tanks (where I like to see roots against the glass), in many of my tanks you wouldn't see substrate at all. So here's the question: Am I required to rip up the carpet once a year, to get at the substrate? If not, how can I accomplish the recommended deep substrate vacuuming? (Subquestion, for extra credit:) Since I use a somewhat labile substrate, should I remove, say the peat, when I'm doing this? If so, should I replace it or not?

Thanks all,

Darrell
 

Tom Barr

Founder
Staff member
Administrator
Jan 23, 2005
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Re: Organic carbon, redox potential and substrates

What do roots add to the substrate?

O2.

O2 + carbon=> CO2

Carbon + no O2=> you get the other reactions and lowered Redox.

A well established root system that you intend on leaving in place generally can go a long time, but you end up needing to make sure you add enough water column ferts and thinning/trimming them up here and there.

Over time you will get sour spots in the carpet, you might not care, but most folks pull that area up, vacuum deeply in that area, take a few sprigs and fill it back in.

I think that is likely more your style for a long term solution.

ADA would rip it up and redo the whole thing.
I likely would also, after 1-2 years, I get tired of a scape.
Many don't.

The summary is about right.
You don't replace the peat, it's for the start up only for the most part till the bacteria are more established.

Once a substrate is mature, 6-12 months in, you no longer need to add anything.

Regards,
Tom Barr
 

dkfennell

Junior Poster
May 14, 2005
10
0
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Re: Organic carbon, redox potential and substrates

Let me ask one more question, if you don't mind:

Given that Fe+++ is a "readier" electron acceptor than SO4-- (and when present makes the environment have a higher redox potential), wouldn't it make sense to seed the substrate with iron to ensure against the reduction of sulphate, by, for example, lining the bottom of the substrate (where anaerobic conditions are more probable) with brickstone, laeterite, clay balls with chelated iron, etc.? I realize that that iron will eventually be used up if no other steps are taken to eliminate organic carbon, but as insurance isn't it wise to do this?

Darrell