Well, after 72 hours of total darkness in my 4' aquarium I removed the
covers and:
thankfully, the fish are all still alive,
most of the plants are fine (java moss has suffered),
a bit of the algae has gone, but most is alive and healthy.

So to those advocating 72 hours of darkness as a guaranteed algae cure I
say: Nonsense, it is a myth. Don't bother even trying it. I did, and it
doesn't work.