Hmm. Then I would go there, get the sickest and line them up, then close your eyes and point. Or just go with your gut. Personally, I would pray and ask which one needed my help the most, but since I'm a Christian and bring that into every aspect of my life, most people aren't going to do that lol.
That was how I chose my Jalen though. I prayed that I would find the fish that was most in need of care, and it suddenly occurred to me to check behind the tanks they had on display right next to the bettas. I did, and there was Jalen, gray as a ghost and literally breathing his own poo. I snatched him up in a heartbeat.
If they're all equally sick, maybe you could just chose the one you think would color up the best once they got better.