Can I use APT START with an inert substrate?Updated a month ago

Yes, with smaller grains (less than 2mm). 

Coarse grained inert substrates such as pea gravel have no ability to bind nutrients - any root tabs or substrate fertiliser will quickly diffuse into the water column. Adding root tabs in such a scenario will only create the illusion of root feeding plants - the aquarist will only be fertilising the water column. 

Smaller grained inert substrates build up organic matter over time and the slower water circulation through the substrate allows some substrate nutrients to be trapped. APT START contains organic matter and slowly releases ammoniacal nitrogen to enrich substrates. Ammoniacal nitrogen binds to organic matter and aquasoil/soil. 

APT START can be used with limited effectiveness in smaller grained inert substrates, but should not be used in coarse inert substrates such as pea gravel.

Substrate fertilisation alone will never make an inert substrate equivalent in performance to aquasoil/soil. The differences in texture and composition that make up soils are very different from what inert substrates offer. For many species that do particularly well in soil based substrates, inert substrate + root fertiliser will not provide equivalence.