If you're on a budget, then i'd suggest do what I do and use a layer of cheap loam/laterite as the base. Cost RM5 per bag, and you only need about 2 bags to fill a 4 footer. The thing with the gravel you are using is there's no nutrients in it, and it certainly helps growth to have a rich base substrate which also encourages plant to root.

Water Lily loam is safe to use in aquaria and i've successfully grown plants in just sand and water lily loam. The gravel you're using is fine, but will be a bit difficult for smaller plants like Glossostigma elatinoides to root and spread. If you intend to grow plants of the Echinodrus family, water lily loam and that gravel you have will be enough to grow giant specimens in no time providing you have enough lighting and co2.

If you need to know where to get some loam, let me know.

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