I thought about the the organic potting soil, but don't know how that would work with my substrate. I have the substrate from aquarium plants dot com. (I got free from them) just not sure if it would be a good cap. I think is size is smaller than eco-complete.
You would have to provide a bit more info or picture on the substrate, as I am not familiar with it, but website appears to say:
Granular Size: 1.2mm to 2.8mm
If that's correct, it should work just fine.