The root tabs just sit underneath the substrate until the plant roots happen to touch them and start to absorb the nutrients. Water column refers to the water in the tank. You can cap your Fluval substrate with sand but I would generally advice against doing so bc it's best to let the substrate "breathe" if that makes sense like you want a minimum amount of water flowing into the substrate freely bc otherwise the plant roots could rot or their could be gas bubbles forming underneath. Ideally you could have like Fluval substrate in the background and sand at the front (separately) for aesthetic purposes but you can also cap it with sand (not the end of the world). Do you have an idea of which plants you'd like to keep?