The thing with Red Tail sharks is that they can be very aggressive towards members of their family. If you have small fish, this is probably what you are seeing. It is recommended to only keep one of these fish in a tank unless it is a very large/long tank with a lot of hiding places for the fish to use. It is also not uncommon for these sharks to harass any other bottom dwelling fish as well so you may want to keep that in mind if you see it/them going after any fish like catfish or other sharks in your tank.
If I recall right (TFH magazine had an article on breeding these many years ago), breeding doesn't start until the fish is at least 4" long. If yours aren't close to this size, it's fighting, not flirting
Hope this helps