The flourite just offers some extra minerals. If it's a fund thing, you could always use just regular aquarium gravel, but finer. The finer gravel is what offers the plant's roots some purchase...those pebbly types of gravel that are great for frogs just don't do that. Also, some people use cat litter for substrate...it must be odor/perfume free, and non-clumping (of course!) That would work in bags. And Tods is right...you can always use root fertilizer sticks/tabs if you find you need to.