If your T5 lights include individual reflectors for each bulb you have as much light as you can use, if not more, without using CO2. It is more than I would consider to be low light.
I think your problem is with fertilizing. Hagen nutrafin plant gro is a trace element mix, with more iron in it than nitrogen, and no phosphorous or potassium in it. You need to be providing NPK, nitrogen, phosphorous and potassium to the plants in addition to the trace elements. You can use basic chemicals, KNO3 (potassium nitrate) and KH2PO4 (mono potassium phosphate) for those, or you could use Seachem's Flourish Nitrogen, Flourish Potassium and Flourish Phosphorous. Without all three of those nutrients the plants will behave much as you described.