HERE'S SOME ANSWERS TO MANY OF YOUR COMMON QUESTIONS
If your tank is really overwhelmed with algae, go to the bottom of this page for my recommended overhaul procedure
This is a bad growth of phytoplankton. It generally starts for no specific reason in both and and fresh tanks (more often in freshwater). Algaecides sometimes work but can be hard on your tank. The most efficient and least detrimental to the ecosystem is to use a UV Sterilizer. We rent "hang on the tank" models that need to run about 10 days to completely eliminate the problem. Once in a while you have the phytoplankton return and a UV sterilizer needs to operate at least 3 weeks then. All the water changes in the world will not make it go away nor will turning out the lights.
If your tank is really overwhelmed with algae, this is my recommended overhaul procedure