[mariannep]

IMO it's not the capping layer what prevents water from being stained by tannins, rather it's the tannins being there and perhaps the water chemistry. Tanks have many factors to take into account. There are uncapped soil tanks with clear water and tanks with tannin tinges and no soil.

I do agree that soil exploding points to specific problems, most likely with anaerobic bacteria (too deep a layer of something). I'd think that solving that should be the first issue.

Cheers!