Nice tank and reds .. as to your question... I have been keeping piranha now for a little over 10 years... Even to this day some of my breeders still get a little skittish after i turn the lights on.. .. don't worry as mentioned above , they will settle down after a while .. adding live plants is a must (Amazon swords) for helping your reds settle into your tank...
also .. piranha are a great fish for learned behavior.. What I mean by this is ... a great way to get your piranha to recognize you is to feed them the same way ever time
try walking into the room slowly
Allow them to see you with out startling them
sit down on the floor in front of the tank slowly
open the hood of the aquarium and hold a thawed shrimp in the water.. Dont drop it in just dangle it so the smell from the shrimp gets in the water.. then take it back out and sit back for a sec
you will see a few moments later that they start swimming around the tank searching for the source of the smell
the longer the period between feedings the more aggressive they will search.. At this point in time they will tend to eat from your fingers.. but be careful while doing this
piranha can jump .. the longer you do this.. the better they will remember .. ie learned behavior
here is a video I made for just this point
hope this helps a little
ps the video is a little large.. but my server has realy good bandwisdth .. so if you have highspeed .. it should only take about 6 min to download.. click save, not open because the file is so large
enjoy
http://www.DontGoInTheWater.com/forum/Videos/Video005.avi