Really, the easiest thing to do would be just letting it get direct sunlight. Phytoplankton is about as versatile as it comes when discussing wavelength absorption, so anything strong, with plenty of blue and red (the latter, more so than the former) will encourage booming growth. A plant bulb would be fine, a 10,000K bulb would be fine, and just about anything in between would be fine; I'd avoid extremes, like actinics, though.
