nematode,
I considered a large skylight. Certainly that could be made to work. I chose the tubes for ease of construction and maximum light delivery. My trio of tubular skylights allow rafters to run between them, so the maximum span that has to be framed is 2'. A 6'x2' skylight requires a much more complicated structure. The labor savings is substantial... and I think you'll find a 6' traditional skylight isn't cheap to begin with.
Tubular skylight surfaces reflect around 95% of the light that hits them. I believe that's much better than you can do with paint. Most of the time the sunlight will be entering at an angle, reflecting off the walls multiple times. The difference between, say, 90% reflection and 95% reflection can be quite substantial after half a dozen bounces. Thus, I believe tubular skylights deliver more usable light than equivalent traditional skylights.
Finally, consider that my trio deliver light fairly evenly across the top of the tank. A single large skylight will tend to favor one part of the tank over another, varying with the time and season.