In a low light setup you don't always need to add ferts; depends on your plantload vs. bioload. If you do get any deficiencies they won't happen as quickly and you'll have more time to address the issue (plants simply grow more slowly in low light tanks, which generally gives you more time to notice and work on problems). Since the plants aren't growing as quickly they don't need the massive amounts of nutrients required in high light tanks.
If you want to read up some on the chemistry of plant needs, etc.
www.RexGrigg.com is a good place to go. Just keep in mind that there's a big difference between low and high light tank setups, and low light tanks don't typically need to supplement or inject CO2.
All my tanks are low light, and I personally use substrates that contain nutrients (I personally like Flourite, looks to me like you have EcoComplete in your tank which should work fine), fert tabs under the plants, and occasional Flourish comprehensive liquid for trace.