# Deep Substrate Bed



## mwhite4823 (May 4, 2005)

I have two large planted tanks with a substrate of Eco-Complete over a 1" sand bed (for the subsoil heating cables), and a layer of stream gravel on top for looks. I constructed both within a few months of each other and the only difference in the setups is the depth of the substrate bed. One has a total of 3 inches and the other about 6 inches (This was the first one and I got a little carried away.)

The difference in the two is now obvious. The 3" one is a beautiful planted tank with healthy landscape and fish. The 6" one started declining after about 6 months. While the water parameters remain good, I can't keep rooted plants alive in it.

I am pretty sure that the 6" has an anaerobic layer. I have never smelled hydrogen sulphide, but I frequently see bubbles rising from the substrate, which I infer must be nitrogen. I do not understand however why the deep anaeroic layer should be killing the plants. A natural pond also has such a layer and plants thrive.

Am I missing something?


----------



## Laith (Sep 4, 2004)

Welcome to APC!

This is a strange one.

Yes, 6" is probably more than you need but shouldn't cause any issues in itself. 

I'd say it has something to do with having a sand bed under so much substrate. Perhaps the sand bed in a 3" substrate is not an issue but with 6" it causes problems?

Who told you that you need a sand bed for the heating cables? Never heard that one before.


----------



## Zapins (Jul 28, 2004)

It might be that in ponds there are many different kinds of substrate organisms that keep turning it over and exposing the soil to oxygen saturated water. 

I bet if you added some MTS' (Malaysian trumpet snails) you would see huge improvements very quickly. The first sign of improvement would be no bubbles.


----------



## bencozzy (Jun 2, 2006)

sounds like the denitrifying bacteria are starving the rooted plants of nitrates.

are you dosing ferts? and what is your water column level of nitrates?

what is your fish load like?


----------



## Zapins (Jul 28, 2004)

It is more likely that the bacteria are starving the roots of O2. Roots need to take in O2, I believe this is one of a plant's primary methods of absorbing O2. If no O2 is left in the soil for the plants to absorb, then the roots will die as 
mwhite4823 is experiencing.

There will always be nitrates in a soil layer of that thickness, unless it is removed physically via water changes or trimming of plants/removal of soil. Even if the bacteria were to absorb all the nitrates in the soil, they are not immortal and when they die, they will be decomposed to produce nitrates.


----------



## bencozzy (Jun 2, 2006)

if the roots arent turning black then its not o2 starvation.

the way you describe it and the only way to see if its nitrate you add kno3 and if you see an almost immidiate increase in buubles from the substrate then its denitrification occuring to fast for the plants.


----------

