Territorial behaviour
Aggression is the word the hobby uses for several unrelated behaviours — defending a spawning site, defending a feeding station, competing for a mate, and hunting — and lumping them together is why so much stocking advice fails. What matters is what is being defended, against whom, and when.
Short answer
Territory is a defended area with a shape, and the shape is what decides whether a tank works. Two fish that both defend the floor conflict; one that defends the floor and one that defends the surface frequently do not.
Four kinds of aggression that need separating
AquariumHQ stores the first three as separate graded fields on every organism, and the fourth as the predation block, precisely because the remedies differ. A record showing strong breeding aggression and no territoriality describes a fish that is a reasonable tankmate for eleven months of the year.
| Kind | Directed at | When | What reduces it |
|---|---|---|---|
| Territorial | Anything entering a claimed area | Continuously, once established | Broken sight lines and enough area for a second territory |
| Breeding | Everything near the site | Only while spawning or guarding | Nothing, while it lasts. Plan for it instead |
| Conspecific | Its own species, often males only | Continuously | Group size, sex ratio, or keeping one |
| Predatory | Anything that fits in the mouth | Whenever the animal is hungry | Nothing. This is feeding, not aggression |
Territory has a shape, and the shape is the stocking rule
A shell-dwelling Lamprologus defends a few centimetres of sand around a snail shell. A red-tailed black shark defends a patrol route across the whole floor. Both are recorded as strongly territorial and they need completely different tanks: the first needs floor area and spare shells, the second needs length so that another bottom-dweller can be somewhere it is not.
This is why volume is such a poor stocking guide for territorial fish. Adding height to a tank does nothing for a species that defends a footprint, and adding length does nothing for a cave-holder that needs a second cave out of sight of the first.
- Floor-defenders need floor area and sight-line breaks: most cichlids, loaches, plecos, sharks.
- Cave-holders need more caves than fish, spaced so that one cannot be seen from another.
- Surface-holders need surface area, which a tall tank does not provide: bettas, gouramis, anabantoids generally.
- Rock-face holders need vertical structure: julies, Altolamprologus, cylindricus.
The Rift Lake exception, and why overstocking is advised there
Mbuna tanks are routinely stocked far more heavily than a community tank would be, and the reason is not that the fish need less space. It is that aggression in these species is distributed: a dominant male with one target pursues it to death, and the same male in a crowded tank spreads the same behaviour across a dozen fish, none of which receives enough to be worn down.
The technique works and it is a compromise rather than a solution. It requires filtration and water changes proportionate to the stocking, and it fails badly if the group shrinks. It is also specific to species that evolved in dense competition on a rocky shore, and applying it to a Central American cichlid tank produces a different and worse outcome.
Reading a fish that has already started
- Watch the fish being chased, not the one chasing. A subordinate that has stopped coming to the front at feeding time is in trouble weeks before any damage is visible.
- Colour is a mood signal in most cichlids. A fish that has gone dark or unusually pale is signalling rather than ill.
- Rearranging the hardscape resets established territories and is a genuine intervention, not a folk remedy. It also resets them for the victim.
- A fish that has begun spawning will not stop being territorial because the tank is crowded. The only reliable options are more space or fewer tankmates.