If just want to use gravel such as small stones is alright in aquarium. I know some people they r scared of cloudy aquarium even just after setup. Another way is not to use any kind of soil. Just go and buy those pellet which u can stick into the gravel such as Tetra Initial Sticks. It contains high Iron, which is sufficient for most aqua plants for start. It also come together with a few pills which enrich the growth of the plants root. It is not expensive too, cost around RM10, which last very long too. But of coz, don compare this solution wit the effect of soil for healthy aqua plants growth. It is still better to invest in good aqua soil, to get the best growth provided tat ur budget is there.