I always run my power heads 24/7. I would not suggest shutting them off at night, and I've seen critters go inside the power heads when they are off, and get mangled when the wave maker turns them back on. You might want grates over the impellers, like the koralias offer.
I run six 2400gph power heads, plus two 1400gph power heads, and 1800 gph return in my 300, and I'm wishing I had more flow.