I really don't see what harm sunlight could possibly do to any of our livestock, unless your system is loaded with deep water critters. It's what all the animals we keep under our artificial light evolved with over thousands of years. There are plenty of threads on various forums, more so based in southern climes, of folks using sunlight exclusively for their marine fish tanks. If not mistaken some of the coral aqua-cultured for our hobby in Florida and/or Hawaii is in tanks under open sky. Search RC for sola tubes & there are multiple threads on people using them to light their tank. I've considered punching a hole in my roof & installing a sola tube for a future tank build, but during winter it would obviously not suffice, days too short & overcast.
Depending on the time of year, I get the odd ray of sunshine hitting my tank. The difference in light intensity & colour compared to artificial light blows me away every time. Here's a shot made in 2010 of the upper portion of my hammer coral being hit with sunshine. I don't have a great camera or the best photo skills, but the difference is quite obvious between the sunlit part of the coral & the remainder. This was under MH/T5HO lighting, I currently run diy LEDs, but there's still no comparison to real sunlight. I think we've become accustomed to seeing crazy coloured coral under artificial light. In the photo the sunlit part of the hammer doesn't look all that appealing compared to the cool greens of the lower part. It actually looked quite nice in real life, the photo's colours are a bit washed out & off. I'm not a diver & haven't seen a real reef personally, but some of the photos taken by pros show a lot of brown coral. I suspect that's the bigger reason most of us prefer lighting our systems under expensive light fixtures, not because of any potential harm from sunlight.
