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Undergravel Filter Substrate (activated carbon? clay?)

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I need to redo my tank's substrate because the tank's pH is too high. The undergravel filter has a powerhead. I was thinking of using a bottom layer of activated carbon topped with porous clay pebbles. Has anyone ever tried something similar? Or know of probable complications? Thanks.
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Hi @MassiveDynamic17,

I use calcined montmorillonite clay substrates with an undergravel filter in two of my tanks with success. I don't use it as a layer, the entire substrate is calcined montmorillonite clay. I used to use Soilmaster Select however it has been discontinued, now I use Safe-t-sorb (40# for $6.49 at Tractor Supply Co). As with most high cation exchange capacity (CEC) substrates it will 'strip' the carbonates from the water resulting in a drop of pH, sometimes a very substantial drop. With my very soft water it drops my pH from 7.0 (degassed) to below 6.0 and result in my having to add sodium bicarbonate (baking soda) to bring the pH back up.

How long does the carbonate stripping / pH lowering effect last? It depends upon the water and how much carbonates are in it and how often water changes are done. In my case, with weekly water changes and adding substantial amounts of bicarbonate of soda once or twice a week the effect still lasted for about 4 months.

75 gallon with Safe-t-sorb substrate (do some research here on TPT about Safe-t-sorb (aka STS) cleaning before use)
 

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Discussion Starter · #3 ·
wow, thanks for the info, Seattle_Aquarist! i was looking for a stone that would help reduce pH.

i hope i can find that stuff somewhere near me. on ebay, it seems massively overpriced compared to the figure you gave lol
 

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Hi @MassiveDynamic17,

Just out of curiosity, what was the pH prior to using the (Calcined) Montmorillonite Clay as part of your substrate?
 
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