About 3.5". My male and female have almost been fighting the past week or so, but it never seemed dangerous. They'd charge each other, and stop a couple inches away, then one would swim to the other side of the tank. Just today though, they're sticking together more than they ever have. When the female leaves the caves, the male will follow her until she comes back. It's fascinating to watch, and they're great fish. Easily the most entertaining in the tank (well, other than when the bristlenose decides to get algae on the top of the plants and falls off).
