The substrate you used is probably eco complete like
A
adamsfishes
says. It is an inert substrate, the idea is that it's porous and will grab on to fertilizers in the water column and then slowly release them. Not my favorite substrate because unless you do fertilize, it's basically gravel. But I've used it in the past as well (before the advent of aquasoil-type substrates) and it seemed to work fine for some plants.