As I understand it, Eco IS inert, but with a high cation exchange capacity to bind nutrients for plant use. I can't see how using it as a cap would be any different from using it for the entire bed in terms of "fish health."
There are loads of tutorials online which explain the mineralization process with how-to steps. The premise is that organic nutrients are converted to a inorganic form through repeated wetting and drying with exposure to oxygen. This allows bacteria the opportunity to remove or convert excess organic compounds which could contribute to nutrient spikes, excessive algae growth, etc.
However it seems as though there are plenty of people having good success using potting soil and similar dirts with loads of organic content.
I'm planning a tank build using MTS with an Eco cap and would be interested to hear others' experiences with growth, dosing and maintenance. The fish, I'm confident, will be just fine.
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