I thought I had read not to put Eco over existing gravel. If I break down the tank and then put the Onyx down with Eco on top, would that be o.k.? Any exp working with both? I already have the Onyx, thinking I want to go black.
IME, layering is a very short-term practice. Sooner or later it is a blend of the two, so if you want a mixed substrate, do that and mix. The finer particles will migrate down and the larger upover time.
The slat and pepper bit would bother me I think, so my first and main objection would be color mix - Eco-Complete is a lot darker than Onyx Sand. The particle sizes of the two are not that far apart, but both do self sort over time.