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In the 'On substrates...' forum Art mentiones that he uses 3-4 inches of Akadama to top the rich organic layer (peat, DYI Power Sand, etc.).
The Fertiplant instructions suggest a cap of 2 inches.
Steve's article suggests 1 inch cap.
Vectrapoint suggest 2-1/2' (front) to 5 inches (back) - http://www.vectrapoint.com/main/manual/bms1.html

Does the thickness of the top substrate depend on the amount of organics in the lower layers?

Does the thickness really matter? Would it be correct to say that as long as the substrate doesn't become anaerobic we are fine?

Are we striving to maintain a substrate that is aerobic from surface to bottom or one that is a mix - aerobic in some areas (organics layer) and anaerobic in another (Iron rich clay bottom layer may be?)?

--Nikolay
 

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The amount of Organic matter does relate directly to the redox levels in a substrate.

I add a little bit of peat in my new substrates for keep the Redox down some. I add more peat and OM to non CO2 tanks becuase I am not going to disturb the layers very often.

If you have a high growth rate, I think the layering issue becomes a problem due to replanting.

As soon as you add roots to the equation, all that crap about redoc and anaerobic conditions goes straight out the door, roots aerate the substrate very well.

So do not put too much into that.

Too much OM can sour the tank's substrate also.

The porous nature of most of the substrates out these days allows you to disturb the substrate yet maitain the anaerobic and aerobic fraction inside each gtrain.

That is much more important than the layering issues.

Total substrate depth is important also.

Generally we put the dirty stuff lowest so we get a little benefit from it without making too much of a mess.

I think the idea of a cap is to prevent the materials from getting into the water column, but the dissolved nutrients will come up no matter what type of substrate is used.

I've found onyx sand and flourite to do very well, yet they are radically different in terms of grain sizing and aerobic capping.
I've also done well with cables and RFUG's.....but the flourite type substrates perform significantly better

I think the real issue is the iron and porosity, beyond that, I think much of it really is not that significant.

A little peat/mulm is all you need, if you want to try more or different types of OM, there's nothing wrong with it although I dount you'll see any difference, and you will want to be care fulm to make sure the difference is attributable to the change alone, that's very tough to do with substrates, my advice start clean and add to it and give it 6-12 months.

Not something you will see all too soon:)

Regards,
Tom Barr
 
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