The bulbs in that fixture are placed very close together, side by side. The fixture is about 5 inches front to back, but only about 4 inches of that is the reflector, if that much, and the bulbs are almost that wide side by side. The tank is 18 inches front to back, so you must have low light near both the front and rear glasses. The tank is also 24 inches tall, so the lights are about that far from the substrate. My guess is that you are getting 50 micromols or less PAR at the substrate directly under the fixture and less than 40 micromols at the front and back glass, both being low moderate intensity. Higher up in the tank the difference between the intensity directly under the light and at the front and back glass will be much greater. I can't tell what plants you have, but if they are high light plants you don't have high light. That could be part of the problem.
Since your plants are pearling you may have enough CO2 for the light you have. Are you using 4 dKH distilled or DI water in the drop checker?
Seachem's liquid fertilizer recommended doses are for relatively low light tanks, so you may have enough, if you are using the nitrogen, potassium and phosphorous ones, along with Flourish, for trace elements.
I do agree with those who say you shouldn't run the lights as long as you were. 8 hours or even less is generally long enough.