Josh, run your tank over night with the skimmer (if you have one) and pop the fish in when you can see the rocks. The only concern would be the fish panicking and swimming into a rock or the glass if it was really soupy in the tank. If you can see the outline of the rocks, they should be fine.
When I upgraded, everything went back in as soon as the rock and water were in. Everything was fine. Don't worry so much
