In a completely pristine tank of saltwater - one that contains absolutely ZERO nutrients, no nitrates, no phosphates no life forms - the sunlight would be fine to see into the tank - but even then - I'm afraid algae would still begin to grow.
Just wouldn't grow very much or very fast.
In a tank full of life sitting in your home next to a window or under a skylight - not a good idea.
Even though the ocean is pounded with natural sunlight containing all wavelengths of visible and invisible electromagnetic radition - it's so freak'n huge, and for the most part is so nutrient poor - that it's not a problem.
Add some nutrients to the mix, as in runoff from the land, pollution or some lagoons - it becomes algae city.
Same in our tanks I'm afraid.
But ...... and there's always a but ......
Go visit the captive coral growing places, like the one 45 minutes north of me ............ and it makes what I just said sound absolutely foolish.
This place is a greenhouse - but has that wavy fiberglass sheeting/glazing on the roof.
Who'd a thunk it