Hi, I've been reading and learning about El Natural aquaria for the last few months now, have Diana Walstad's book on order

and am planning on re-starting my current 125L as an El Natural Aquarium.
Please forgive me if this question has already been asked, I've searched the forum and can't find the answer.
My biggest problem is finding a soil that is suitable. I did buy a bag of Westland John Innes No 1 (for seedlings and potting young plants, but on further reading it seems that this may cause problems - no matter, if this is the case I can use it in the garden tubs.
Which brands/types of soil should I look for? - and where can I find them?
Does EVERY soil have to be soaked for weeks first?
I have fish and shrimps who will have to go back in the tank ASAP as I have only temporary accommodation for them.
Thank you for reading this and for any replies