Well where do they lay their eggs? I'd consider that home base. They used to host in my Duncans until the Duncan's perished. :(
But most of the time they like to hang out in their little cave with their eggs. They fiercely defend it.
Maybe yours think your tank corner is their cave?
The only time they stopped spawning and defending their 'special place' was for those 10 days that I had a tang.