My tank has gone through some changes lately and we are often seeing new behavior. Of the 18 fish I have in the 55g I have what I think are two male yellow labs and one female. I'm certain about the female because she holds once in a while. But the two males, which often hung out behind the rocks have both started digging in the middle of the tank in front of the rocks and building a cave, but they are doing it together. This seems very odd to me for a few reasons. They are the least aggressive of all the fish, they don't dig (i've never seen them dig), and they seem to be working together...Any thoughts?