Thank You Roadmaster and mgeorges. 850 is way up there. I initially thought of adding inert gravel top off the mineralized soil layer at the bottom. But I was suggested clay pellets to be a better option as it was good for the plants to root in, high CEC, allowing stuff to get into the bottom layer etc. And then when I completed the setup I realized that it is leaching badly.
And yes, I rinsed it very well for two days before adding it. The water and is extremely clear btw. To remove all the plants and replace the substrate would be a painful procedure. But if it has to be done, it has to be done.