# Active Substrate in Action



## jameson_uk (19 Nov 2016)

I went with some Tropica aquarium soil in my shrimp tank but I am interested in what this is actually doing.

The details just say


> Aquarium Soil ensures good and active growth from the beginning, and boosts the red plant shades.
> 
> It is a complete substrate, which can be used without any other types of bottom layer.
> 
> Aquarium Soil is further an active bottom layer that lowers the pH value and slightly affects the water chemistry.


Which seems a bit wishy washy...

Thinking about the KH / pH / CO2 chart and as I guess the substrate can't change CO2 so the only way to lower pH would be to lower KH?  In main tank GH is 10/12, KH is 4/5 and pH is 7.4. Based on the chart this equates to 5/6 ppm CO2.  If the substrate dropped the KH to 1 then pH would reduce to 6.8 based on same CO2 ppm.

When I was starting all this I am sure I read lots about making sure you have a high enough KH to prevent a pH crash so not sure what the impact of this might be?

I am guessing that the substrate won't change the GH leaving this tank with a low KH and high GH?  Could this cause any issues?

Finally the substrate is meant to have a lifespan so how will I know it needs replacing (is it a case of suddenly finding the pH has jumped; and shrimps probably die off...).  If I use my tap water to do water changes I guess this will reduce the life?


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## john dory (19 Nov 2016)

I think the lifespan bit,refers to the rate that the nutrients are used up.


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## dw1305 (20 Nov 2016)

Hi all, 





jameson_uk said:


> Aquarium Soil is further an active bottom layer that lowers the pH value and slightly affects the water chemistry......I guess the substrate can't change CO2 so the only way to lower pH would be to lower KH?


These are active substrates with <"cation exchange capacity">, which means that they can exchange monovalent cations (like Na+) for more strongly bound multivalent ions (like Ca++).

You can have a clay with a <"low base percent saturation"> where the cation exchange sites are filled with H+ ions. When these H+ ions replace metal ions in solution the ratio of H+:OH- ions, that we measure as pH, is switched towards H+ and acids are <"H+ ion donors">.

I would imagine that most active substrates are clays that initially have their cation exchange sites filled with H+ ions.

cheers Darrel


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## jameson_uk (22 Nov 2016)

dw1305 said:


> Hi all, These are active substrates with <"cation exchange capacity">, which means that they can exchange monovalent cations (like Na+) for more strongly bound multivalent ions (like Ca++).
> 
> You can have a clay with a <"low base percent saturation"> where the cation exchange sites are filled with H+ ions. When these H+ ions replace metal ions in solution the ratio of H+:OH- ions, that we measure as pH, is switched towards H+ and acids are <"H+ ion donors">.
> 
> ...



So in simple terms the soil has a finite number of slots to do this exchange.  These slots are one time only and once the slots are all used up the substrate will no longer be active?

If this is the case then will my tap water (GH 12 / KH 5 / pH 7.4) get through these slots in no time if I use that for water changes?


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## roadmaster (22 Nov 2016)

Soil's are so much more than  I had ever considered, and am glad I employ soil  in my aquarium's.
Been reading from an old text "The nature and properties of soils" by Lyon and Buckman circa 1947.
I read till my eye's begin to bleed, and then put it down and return later to read some more.(is good reading for those interested in soils)
 Harder to deplete soil's with addition of fish,shrimp food's,fish waste,, than I first thought .
I would not use hard,alkaline tap water alone for water changes in tank where pH is being influenced downward by substrate/hardscape if stability was my aim.
Would mix Tap water/R/O water to match water in the tank ahead of water changes to use.
Not much trouble for small tank's but a bit more tedious in tank's larger than 200 litres.


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## dw1305 (22 Nov 2016)

Hi all, 





jameson_uk said:


> These slots are one time only and once the slots are all used up the substrate will no longer be active?


Yes and no, it is cation exchange, and the way ions are exchanges depends on abundance as well as valency. 

This means that you can re-charge a calcium loaded ion exchange resin in a strong acid, or brine solution, where the H+, or Na+, will replace Ca++ ions, even though they are less strongly bound. 

For  an active substrate once all the exchange sites are filled with multivalent cations then they are going to stay there and it is no longer "active".

cheers Darrel


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## jameson_uk (22 Nov 2016)

dw1305 said:


> Hi all, Yes and no, it is cation exchange, and the way ions are exchanges depends on abundance as well as valency.
> 
> This means that you can re-charge a calcium loaded ion exchange resin in a strong acid, or brine solution, where the H+, or Na+, will replace Ca++ ions, even though they are less strongly bound.
> 
> ...


I knew I should have paid attention in chemistry rather than just setting fire to things...   Physics seemed much easier!!

I did have to google a lot of that and did come across (http://www.soilquality.org.au/factsheets/cation-exchange-capacity) which just about makes sense
https://en.wikipedia.org/wiki/Hard_water says hard water is the the concentration of these multivalent cations.
Given the tap water GH here is ~12 that puts me in moderately hard / hard water territory.   So the more tap water I put in, the more of these multivalent cations get exchanged thus the more sites are going to get used up.

I suspect I am not going to attempt to try and refresh the substrate with acid so I am basically waiting to measure a rise in pH which will probably be the main indicator that the substrate is no longer exchanging and needs replacing???


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## dw1305 (23 Nov 2016)

Hi all,





jameson_uk said:


> .....says hard water is the the concentration of these multivalent cations.........So the more tap water I put in, the more of these multivalent cations get exchanged thus the more sites are going to get used up.
> 
> I suspect I am not going to attempt to try and refresh the substrate with acid so I am basically waiting to measure a rise in pH which will probably be the main indicator that the substrate is no longer exchanging and needs replacing


Yes, that is it.

cheers Darrel


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