Thanks for the reply, the stripe of dark substate is band new Flourite, that's fine.
I had a small layer (2cm) of pool filter sand, added a layer of Flourite and a top layer of pool filter sand.
The original layer of pool filter is what is rotting. There's a small circle on the pricture and I thing there's more I just can't see.
Thanks for the above post, I did not know of hydrogen sulfide, I'd love more information. If I have to remove the old first layer, do you think I could do it in to months?
I'll remove and save the new top layer of pool filter. Remove and save what I can of the middle, new Flourite and throw out the first old layer.
I need two months to save for another 8kgs of Flourite. It's so expensive in South Africa.