Using an empty tank, with just the substrate in it, to start a carpet of plants like HC changes the "rules" we follow a little bit. Since there is no water above the substrate you don't need to be worried about ammonia or urea, so an ordinary garden type fertilizer will work. You do have to be careful not to over fertilize that way, or it will "burn" the plants. And, when it is time to add water to the tank you need to be pretty sure all of the ammonia and urea havae been converted to nitrates, which means several weeks should pass since the last use of that type of fertilizers. Aquarium fertilizer tabs should work fine too, without the ammonia issue.