the only problem i would see if you have them set up on a timer smaller fish may crawl into where the propeller is and when its time for start up get mangled. i would leave them on. unless you have the tunze or vortech which like previously stated just slows down at night

Since installing a second VorTech in my system I've started using the 'slower' night mode. Been quite a while now and no apparent problems with lack of O2 at night. I wouldn't do it with just one though, don't think it would have been sufficient.

Yes they do :)

Many wrasses for example secrete a mucus sac in which they sleep - helps avoid predation while they're out for the night. I've seen my sixline do it myself - wedged in the rock, unresponsive to stimuli such as a moving feeding stick and a shining beam of light.

yea my valentini puffer goes on a rock and sticks to it and sleeps its quite cool to watch