I bought a rather expensive bag of aquarium soil for my aquarium because I want to plant it. Not sure of the brand - it is in Chinese - but it is micro black pellets that can be smashed into soil easily with the hands when wet. When I bought my plants at the LFS, they showed me how to plant them - take some tweezers, stick one end of the plant into the soil and tamp with a stick or your finger. I tried doing these with my soil and it does not work - I think it is not heavy enough. Should I put a layer of gravel on top of the soil to aid in planting? If so, what size?