I have a big group of BNs, more males than females. They did nothing for 4 or 5 years then didn't stop breeding for another 1 or 2 years then stopped again until another 4 or 5 years went by then suddenly I find fry zipping about again. Of course the fry could be from some of the resulting original fry, not the originals. I have no idea who is doing the producing as I don't know where they lay the eggs. My tank is heavily planted and has no caves, just big bits of driftwood. I suspect they have a hollow area in the driftwood somewhere, or under it. :-?