I am currently rescaping my 75G and plan on using Oil-Dri as an inert base to build height in my tank. It is similar to Safe-T-Sorb. I've read of CEC capacity on baked clay products, as well as people charging it with minerals and ferts prior to use as a substrate to manage the issues that come with the CEC. Since I'm using this as a base/inert layer, I was wondering if charging it was necessary? Or perhaps charging it with only baking soda? The plan is to cap the Oil-Dri with soil, and cap the soil with aquasoil.

Link to journal: https://www.plantedtank.net/forums/1...xperiment.html

On your build cover those drainage grates with a couple layers of fiberglass window screen to keep clay to about 1.5-2” thick all around, that way you won’t have to deal with as much high CEC material. Your soil and AS on top already has enough CEC/buffering capacity to worry about. No sense adding a bunch of high CEC media as a inert filler (basically creating a giant nutrient sponge) that your plant roots will never touch. Best to leave all that space in grates as a empty void. The 1-2” of clay under soil layer that you put down will give you all the anaerobic layering you could need.

Thanks @DaveKS. Follow up question: With a layer of CEC material as deep as originally planned, would the sponge effect be as pronounced since flow will essentially be null at the bottom? I figured the water column would be the same throughout, just thought that since it was so deep, nutrients wouldn't leech into it (in any significant amount) due to lack of flow and "fresh" water.

CEC is attraction of positive charged particles to negatively charged CEC sites in media. Restrictions of water flow will slow it down but it will never stop until all the medias CEC potential is completely exhausted.

On a molecular level it sets up it own water exchange. As minerals are pulled out of solution and bound, the water becomes less dense and rises and is replaced by denser water. Even if you put clay substrate in bucket a couple inches deep and fill bucket with water of a known ammonium or ther positive cation element level, no circulation at all, it will continue to absorb those all on its own until all the CEC potential of clay is exhausted.

Use a large pored substrate like oilsorb or soil balls and that natural water exchange can happen very quickly. Real soil type substrate also contain humates which allow them to also absorb other negative charged anions such as phosphorus and iron. Clay, out of the package can’t, but over time it’s pore structure will absorb humates which will increase its ability to attract anions.

With you planning on putting soil over clay it will build up a humate level pretty quickly.

So I did a bucket test and ran into some surprising results. Definitely does not match any experience I've read about using heavy CEC material. I filled a bucket with 4 gallons of tap water, and poured Oil-Dri in until I reached the gallon mark. Once everything settled, I tested the water and compared it to my control (tap water)

Tap Water
pH: 7.5
GH: 12 drops
KH: 6 drops
Ca: 60ppm
Mg: 15.7 ppm

Oil-Dri Bucket:
pH: 6.8
GH: 38 drops
KH: 6 drops
Ca: 150 ppm
Mg: 74 ppm

Not what I had expected. I thought my GH would drop off to almost nothing, not go the other way! Is my bucket test not a valid one? Maybe I will try again, this time rinsing the media before pouring it in.

That isn't surprising.
Oil-dri is essentially clay

The magnesium is surprising,as the oil dri should "catch it".

However, everything else makes sense. The oil dri grabs a lot of cations, many of them increase pH.

I would expect oil dri to increase TDS. TDS just measures conductivity of water. Oil dri can't remove the conductive elements, only bind them. They are still conductive.

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My mistake. I had remembered all the forum entries incorrectly and was expecting my GH to crater (the forums all mention KH cratering, although mine has stayed the same).

The magnesium "tests" are derived from the equation ((GH in ppm) - (2.5 x Ca in ppm))/ 4.1 = Mg in ppm.

Maybe this is inaccurate?

So how would I go about dealing with all this hardness in a new tank? Do I lay down my substrate and hardscape, and flood the tank with no plants for a couple of weeks? Water changes every day until GH, Ca, and TDS come back into range? I feel like plants right from the get go will not have an easy time.

Fullers earth can have varying amounts of calcite, dolomite or quartz depending on where it is mined. So depending on its source, it releasing Ca and Mg ions while binding others is not impossible.

Did your rinse oil dry throughly before trying this? Repeat experiment and dose it known levels of nitrate and phosphate and see what happens.