Gravel can be used as a substrate, as long as you choose the right size, it is inert (meaning it won't alter your water parameters, and there are no artificial dyes. You will need to look for 2-3 mm particle size (which can be difficult to find) and you will need to fertilize it with root tabs, laterite, or even half flourite. There is a flourite black available that you could use. I actually mixed my tank with half gravel, half flourite to save money.
Edit: I want to clarify the statement above. Laterite and Flourite is NOT a fertilizer, but does have mineral substances such as iron and gas the ability to hold onto excess fertilizers. You may chose to dose with fertilizers as well in the above scenarios. With some tanks, I do not dose at all and others, I dose heavily. It really depends on your expectations, choice of plants and such.
Without Algae, death of mankind would be inevitable.
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