That is entirely up to you and what you want. Some people just use ordinary gravel while others will use a substrate that is specifically made for an aquarium with live plants such as eco-complete or florite. Eco-complete is black in color and is a fine substrate in texture. It comes in a bag with liquid that has live bacteria needed for a planted aquarium. Florite is another substrate made for this kind of aquarium but is reddish in color and the substrate is in bigger pieces which have a lot of iron for the plants. Both of these options are a bit on the pricey side.
looks like you can get the eco complete and just make a small layer and put substrate of choice over that, I will be setting up a 90g, if I were to get 40 pounds of the eco complete and cover it with gravel of choice this would work out ok?
yeah, figured that would happen. Looks like though you can get a layer of that eco complete and put whatever substrate you want over that. I may go that route. would 40 pounds of the eco complete be enough for a bottom layer in a 90g? 48x18 bottom dimensions. then maybe 60-80 pounds of gravel on that?