I was thinking of having grass on the bottom. So would it be alright to just have fertilizer on the bottom? Or do I need some kind of substrate like sand?
I have grown Echinodorus tenellus, Eleocharis acicularis, Eleocharis vivipara and Eleocharis `parvula` successfully in sand, but that is with an EI fertilised water column.
I am also successfully growing Eleocharis acicularis and Eleocharis `parvula in ADA Aqua Soil Amazonia.
Provided you have enough ferts in the water column, you should have no problems with an inert substrate.