Darkness is needed to complete photosynthesis.

[20GT]

Obviously light alone does not accomplish photosynthesis. The light phase does not convert carbon dioxide into a carbohydrate which is an integral part of photosynthesis. Darkness is needed.

What if i create a special box in my filter line to keep light on the water during the night as long as my pump is running.

http://plantphys.info/plant_physiology/calvincycle.shtml

 

[JasonLion]

No, darkness is not required. Several of the reactions are called "dark reactions" because they do not require light, but light doesn't prevent them from happening. In practice they only occur when there is light, since the "light reactions" portion of the cycle does not run in darkness, and the light reactions are required to drive the cycle. See thisWikiPedia article for more information.