Actually it has happened, though only in the downtubes into the aquariums. For some reason though, maybe because of the environment, there hasn't been any battling inside the tubes. There is room for two fish to get past each other quite easily though. The magnification effect of the rounded tube makes it look fuller than it is. If it does happen I'll have to figure out what to do about it though.
As to a territory claiming inside the tube, believe me I've thought about this. So far it hasn't happened, but again I'll have to intervene if it does, through closing off the tube for a time or some other tactic.