First off, I live in a bachelor (aka a very small three room) apartment with a giant south facing window that provides tons of natural light, regardless of weather conditions. My tank light would be on for about 15 hours of the day. I decided to keep the tank light off for a few days and just remember to open the curtains to provide the tank with the most natural light allowable from sun up to sun down (about 15 hours). The tank light would only be on for an hour for feeding, at some point after sun down. I had chosen to do so because I thought the lighting might have been the cause of a problem I was having with the tank. Everything has been fine for the past week or so that this has been done. There have been improvements to my delight in some displeasing conditions.
The plants (Anubias something, and an unknown type) are doing well and the fish are great. Would be just fine to continue this? You can see the amount of light the tank is exposed to on a cloudy day here, http://www.bettafish.com/showthread.php?t=22202&page=13
Is the tank light needed for certain things that sunlight cannot provide?