Because zebs are relatively new to aquariums, they don't have the tolerances of other shrimp like cherrys or even CRS. You need to match the water parameters and conditions found in their natural habitats if you want to have any success with them.
I don't think the aquasoil is a great idea, simply because it messes with water parameters (reducing pH and exchanging ions). More important is stability and low TDS. Forget pH, its just not important - have a look at my field trips - they are found at every pH but TDS is ALWAYS super low.
I use inert substrate - specifically granite which matches the rocks where they naturally occur. I'd suggest you do the same.