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STS, because of its porosity and clay component and high surface area, has plenty of binding sites for nutrients (CEC or 'cation exchange capacity'). It also has plenty of attachment sites for bacteria. If the STS is anaerobic, say at bottom of a deep substrate filled with organic matter, it will support denitrification, an anaerobic process. On the other hand, if the same STS is in an aerobic environment with enough oxygen, it will support aerobic bacteria like nitrifying bacteria. It simply provides attachment sites for bacteria, and since it also binds nutrients (NH4+, K+, etc), it encourages bacterial activity.

Apparently, the thin layer of STS scattered on the bottom of my potted plant tanks stays aerobic, and thus, does not support denitrification. I wasn't sure about this until I did my experiment showing that 10 ppm added nitrates did not decrease over a 13 day period. If tanks were actively denitrifying, I should have measured some decrease in nitrates.

I bought a 40 lb bag of STS for $7 at Tractor Supply Co.
So STS is a geo specific product ? I mean it is more of a brand's name for a product but specific to that brand that may not be available in other geolocation. You describe its behavior but if i were to go to the store it would be next to impossible for me to determine if an alternative product had the right properties ?

Having said this - i have some interest in the topic since I have found that a specific fine inert substrate rapidly produce anaerobic pockets but there is a difference between that type of anaerobic pocket and what you are describing since i find it rapidly kill plants (I've run the experiment several times) - I suspect the difference between this substrate and STS is the clay component allowing for porous property and therefore the anaerobic bacteria you are describing is a bit different. I have another substrate (also inert) that does not have this 'kill plant' property which i have described as porous.

In the first case i am describing above i had a heavily stocked aquarium that I did not perform water changes for 3 months (because it was on the floor and siphoning water out was difficult) but the nitrate level stayed below 2. This was an inert substrate - but instead of releasing sulfur (egg smell) i applied a gentle current over it and it release (I think) nitrogen and very small patches of cyobo developed on it - the positive effect of this gentle current was that the plants grew instead of dying.
 

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My biggest concern in this thread is introducing toxicity to the tank. When I read the above past about clay contain aluminium or iron it just makes me thing I should stick to inert substrate. The plants might not grow as well but they grow and the fishes don't have to worry about long term poisoning.

I think the biggest question i have given the above comments on clay is how do you know if any of the products are safe for long term use. I realize this thread doesn't seem to focus on fish health but isn't that a concern when picking your substrates ?
 

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I'm not disagreeing or agreeing but the post above mine by Diane has:

"Remember that clay is composed of aluminosilicate, meaning that all clays contain aluminum and have the potential to generate aluminum toxicity."

which seems inconsistent with your statement or perhaps i am misunderstanding something...

Calcined clay is similar to porcelain and people have been keeping fish in porcelain bowls for a thousand years. I can't speak for the chemical properties of Matrix and Denitrate except to say that Seachem advertises them both as being "inert".
 
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