Okay, Flourish Comp is micro-nutrients only, juts FYI. Which "Nitrogen and Potassium supplement" did you get? And what about Phosphorus? You might not even need ferts at all, depending on your light.
The light is the stock that came with the cube, how many watts is it and what kind of bulb (curved thin tube (PC) or straight bulb (t-5 or t-8 ), etc... )? And what substrate are you using?