If the fluval plant sub is anything like their shrimp sub, it will not work for HC. The FSS is really light, and plants easily become uprooted, keeping anything smaller than chain swords and crypts down is a chore, boyant plants such as erios will be popping out of the substrate daily. I tried the MGOPM capped with river sand in my new tank and planting in it is a piece of cake! But to answer your question, you should add some ferts, I'd go with root tabs. If I'm not mistaken fluval stratum provides no ferts.