I was doing a large water change today of nearly 80% and as the tank was refilling I realized I forgot to add Prime. I noticed my two Uaru hiding in a corner as they sometimes do during a water change, but they looked more stressed than usual. I caught my mistake once the tank was probably 30-40% full and added twice the Prime this time. All fish seem ok only a few minutes after the tank has been filled. I'm concerned about what I might wake up to in the coming days though. The filter was turned off. What damage might have been done to the fish/their gills?