The title of this post is a bit misleading. What I mean by “building” isn’t exactly synonymous with turning raw material like clay, for example, into something that resembles a cave. This post is more about using various aquarium-safe objects for caves and breeding structures.
Territory disputes? With both species only and community tanks, this can be a potential problem. It can be mitigated by overcrowding, i.e., having so many fish that no single fish can claim a territory. Even if your tank isn’t overcrowded, you can still sometimes ease such disputes if you move things around in the tank frequently (e.g., rocks, caves, decorations).