I don't think this has as much to do with the constant light as you
might think. Was anyone feeding your fish while you were gone? If so
were they feeding them as often as you would have?

My guess is that for 3 days no food went into the tank so the plants
cleaned up the water and then had nothing left to clean. The lack of
nutrients stopped plant and algae growth and cleared up the water. Also
the fish are probably moving around less, thus stirring up the water
less, thus no tiny bits floating in the water.

The constant lights stressed the fish a little bit, but not too much.
The added stress caused the rasboras to school together which meant
they stopped picking on the runt. The runt, who was over stressed
before, is now a little stressed by the lights but all in all the
reduction in stress resulting from no longer being picked on while you
were gone caused his overall stress level to go down - thus he looks
better than when you left.

So what have you learned? Stressed underfed fish don't fight or pollute
the water as much as happy well fed fish do. You have unlocked the
secret that your LFS uses to keep massive amounts of fish in small
spaces without having them die ;-)

Seriously though I think this is possibly evidence of the fact that
your runt is getting picked on - you may want to try and do something
about that. I haven't any suggestions as to what since I've never kept
harlequins.

Neat experiment. Even if it was accidental.

-Daniel