Hi @Hanuman , Yes, my inert but "mature" substrate is an unknown factor. I did grow plants in it for 2 years. Previously I did dose quite heavily (EI levels) for a long time. I do not know what level the substrates Cation Exchange Capacity is at - thats a big unknown. I mostly kept the substrate undisturbed, with only occasional vacuuming of mulm and detritus. Also, I am using a bit of Osmocote balls in the area where I keep the stem plants. I hope I do not have to replace the substrate... Lets see. It might be feasible to deploy your idea of gradually replacing the substrate with Amazonia.I suspect your inert 2 years soil will be far from enough. You grew plants during these 2 years yes? Unless you didn't and dosed heavily and your substrate has a high CEC those plants also sucked up some of the nutrients so your soil will not be as heavily fertile as you think. Note that Sudipta used Amazonia light then also added some Amazonia I. Those are heavily fertile substrates.
In my tank, I can tell you that after 1 year using Amazonia + Black Earth I could see some plants struggling and instead of using osmocote or the like I simply sucked sections of substrate and added new one. Within the next month plants in those sections would rise to fame.
Anyway, you got to try else you'll never know. And yes please do that journal 😬