The substrate Jbrofish8 mentioned does indeed look lovely in his tank photos. If the only fish planned are what you (Gwen) mention, this is a good choice as it would appear quite natural. There are also other black sands/fine gravels, they are quite expensive.
As for an enriched substrate, these too are very expensive. I have Flourite in my 70g and so far cannot recommend these heartily. For one thing, substrate fish are a problem, they are sharp grained (both Eco-complete and Flourite are, other brands may not be, I've not seen them first-hand). Second, I am not convinced of the extra benefit for the plants over sand or gravel, and this is after more than 6 months.
Having prepared new gravel, playsand and Fourite during the past few months as I reset all my tanks with new substrates, I still prefer playsand not withstanding the extra rinsing. It is cheapest by far, looks natural, is inert, plants like it, fish like it...what more could one ask for.