My experience has been that lighting was more important to plant growth than the substrate, but then I have mostly low light plants. Many of your more high tech. tanks use flourite as part of their substrate, but it can cloud your tank easily. Eco complete would probably be the easiest plant friendly substrate to use because it contains a lot of the nutrients the plants need. Be careful with ferts also. It can increase algae growth if you are not careful. I have low light plants, and very rarely fertilize. When I do add ferts, it's root tabs.
There are various kinds of substrates, depending on the types of plants that you have. In a planted tank there are 3 major considerations to take: the plant types, the substrate, and the lighting. The substrate and lighting should be matched to the types of plants that you have. There are low-, medium-, and high- light plants. Low-light plants usually don't require elaborate lighting and substrates nor fertilization. Some medium-light and all high-light plants will require good substrate, high lighting, fertilization, and even CO2 injections. There are some plants that don't require any substrates at all because they grow attached to surfaces such as driftwood and rocks - these are usually low-light plants. As for the nutrient-rich substrates, I hear all the time that Seachem's Fluorite and CaribSea's Eco-Complete are best. Fluorite can cloud the water a lot initially, so you need to know how to rinse it properly before adding it to your tank. Here is a good article on how to rinse Fluorite:
Keep in mind that the higher the lighting, the more ferts you'll need, and possibly CO2 injections. (And as Gunnie said, with lots of ferts you're risking algal blooms.) That's because higher lighting causes the plants to metabolize faster (use the available nutrients faster). And the faster the metabolism, the more CO2 the plants will need. That's why I'd recommend a plain low-light tank for beginners wanting a planted tank (this is how I'll set up my planted tank: low-light, low-tech).
That's about 1.50 watts per gallon and those are pretty well low light plants. I would say you have a low light tank from what you tell us. However, the unknown plant and the grass may need more light. Without knowing what they are it's hard to say. Do they all seem to be growing well?
Well, if you want a medium-light tank, you have to be careful with wattage because anything around and above 2 watts per gallon will also require CO2 injections. The more the light and the stronger the light, the faster the plants grow. Which means, they need more CO2 under high lighting. So if you want to have a low-tech tank, without CO2 injections, aI'm for low light and for low-light plants. A low light tank, I suppose, can have anywhere between 1.0 - 1.8 (2.0 max) watts per gallon of light (though I'm not sure if 1.0 isn't too low). If you want to figure your lighting, divide the total wattage over your tank over the size of your tank. That will tell you how much lighting you have over your tank. If you think it's too low, you can either replace your current fixture for another one, or add another fixture to the existing one.