If you intend to grow marine algae for nutrient export then you will want a light. If you are using the
sump simply to increase the volume of your system then you don't need a light. Many reefers grow various marine
macro algae in their
sump and put a light with a timer on a reverse daylight setting to help control
pH swings between day and night.
I have a single bulb standard 16" fluorescent aquarium light that I leave on 24/7.
Dick
