You have alot of options sand or gravel, soil capped with another substrate, and many choices of commercial substrate. Capped soil is a very nutrient rich substrate but in the beginning you have to deal with gas pockets (which you daily need to poke around the tank with a skewer or something to release the gas-this is for maybe 6 weeks). But plants take off in dirt, never have expierenced any type of melt on any plant in a dirted tank. You can also mineralize dirt first and then you don't have the gas issue to deal with. I've heard nothing but good about mineralized dirt. Sand and gravel work but since they are inert and not capable of holding nutrients you need to use root tabs. Commercial substrates like Eco-Complete, Florite, and others have a high CEC which means they are able to hold nutrients and make them available for the plant roots. In my opinion you can make all of them work and it boils down to how much you want to spend.