they wont fuckign burn unless your using 800 ppm in your solution or more wich would burn it anyways lol. They like it in the morning when they first come on and at night when the lights are about to go out.
I do it about an hour before my lights come on so they soak it all up before Mr. Sunshine comes on. I dont like doing it after the lights go off out of fear of mold or something.