How do I get a rich substrate?

Hi!  I have a second tank that I *don't*  want invest in an expensive 
lighting arrangement(my other one has the lights), so would like to get 
plants that do not require more light than comes with the standard 55 
gallon tank setup.  The plants that I am finding that do OK in lower 
light conditions require a "rich" substrate.  I have noticed the mail 
order company "Natural Aquarium and Terrarium" has several items for 
sale...Floredepot long term gravel substrate($26.99 for 26.6 
gals)...and...Hilena Initial D gravel additive($5.99/enough for 55 
gals).  I am not a geologist so cannot get to technical on this 
subject.  Does anyone have any information on these or other "quick" 
solutions so that I can plant this second tank with low-light plants?  
Currently it has plain gravel.