Well I just doubled the number of X-ray Tetras in my tank and the old and new fish are schooling together now. My new saltwater tank is next to it, and while I was checking the SG for it my elbow was wavering in front of the tetras. I noticed the tetras shying away from my arm every time it twitched, so I decided to spook them with my pointer finger (this did not involve me submerging my hands. Only half of them shied away. I figured the ones that shied away were the new ones. For any of you that have just added new tetras and need some way to distinguish between the new and old ones, I hope this helps you. I'm crazy. 
