I'm considering going down the black sand route for a makeover after being dissuaded from using crushed coral. However, thinking about what, if any, nutrient layer I should use I've seen several Tropica products that maybe do the same job. One product in a local shop is called 'Substrate - nutrition for plants'. Is this the same as Aquarium soil? Also, can I use this with sand on top or should I use gravel over it? Thanks for any advice offered.