We have a walstad style low-tech 20tall mounted in a window on the west side of the house which has no lights - just the window. For this tank we have parchment paper and bubble wrap behind it to attenuate the light. otherwise it gets too much light and grows algae.
we also have a 90 gal in the dining room on the north side of the house directly in front of the window. it has CO2 injection and the only lighting is 2x54w T5HO 4hr/day + lots of indirect sun.
additionally, a 10 gal half-full shrimp breeding tank in front of a sliding glass door with a patio roof outside. no direct sunlight light and no artificial light, still grows great.
if you plan things out right, it is possible to have a tank with little or no artificial light.
check out this monster thread on reef central for info about sola-tube lighting.
Reef Central Online Community - SolaTubes for reef tanks