There were several 4" 7 strip frontosas at LFS. I noticed one more dominated over others. Yet they weren't too bad due to more community environment. I always thought if it has more bump on the head on certain stage(?), I assumed it was a male. However, the store owner told me we should judge by the length of pelvic fins. If it is longer, it is a male. I still doubt his theory. Based on his opinion, we bought one with longer pelvic and other with shorter. That afternoon I acclimated both in 75 gallon QT with PVC pipes to be safe. We stepped out for 1/2 hour and came home to check and found one of them was killed by the other(all of the fins were shredded). Next day we went back to complain. Now she told me we should have bought more than 2, or more 4 frontosa because they should be community fish until they get paired up. I still think we were mis-informed about the sexing.