Informational: My fish are fighting! ........Help!

Informational: Help, my fish are fighting...!!!
The old freshwater designations of aggressive and community don't work well as marine concepts.

Marine fish are almost all (including the oblivious-to-the-world mandarins) capable of killing another fish, under one of two conditions: lunch, and if your tank is too small to prevent encroachment on what a fish's biology informs his small brain is 'his' space, or his food supply.

In the ocean, one fish eats another, often whole. Or one fish encroaches on another's territory, and loses the squabble, often without a single nipped fin: he's outclassed, knows it, and leaves. This goes on all day long. When night comes, fish head for sleeping holes---and the same rule applies. If you're a smaller, weaker fish, you don't get a hole (nobody shares,) and you're out there in the dark hoping the night-prowling sharks don't notice.

In your tank, it doesn't matter spit what your future intentions are, that 'much larger tank in the by-and-by'---the little fishy brains MAY be pre-set for a certain size territory around them. Yes, that expands as they get bigger, but the proportion is the same. One day in June your rabbitfish that was fine in April may start killing his tankmates.

Fish may also be completely blind to other species coming and going in 'their' space, but one of their own kind intruding into that perimeter --- WWIII. And death for one, because there is no retreat. The yelllow watchman blusters at everybody. But rarely bites. Put another yellow watchman in, and it's death for one, unless your tank is large enough for mating behavior, or co-existence at enough distance, which with this species is probably yards and yards of distance.

Mated pairs claim, as a rule, double the territory: they may let some species wander through, but to others, they take exception, and mayhem results.

I'd be hard put to define ANY marine species as 'community'. Most will kill, given the conditions above---if hungry and housed with another fish that will fit in the mouth, boink! gone. If 'crowded' in his own perception by a species that he 'notices,' whether it's his own species, or a competion for the same food. You put the wrong species into the tank --- and all of a sudden you've got a war going.

Does it ever stop? IF there's enough room for them to get away from each other, yes, if not---one dies. It's about territory, and until they've worked out the math, they're going to fight every time they see each other. Fish do have 'neighbors,' with 'swim-through' rights, but that develops over time, if they live long enough.

If you are going to put a highly territorial fish into a tank, put him in last, or he'll claim it all.
If you have a fight start you didn't expect, try moving a significant (fairly large) rock. This causes a territory reassessment and may take off the pressure.
If you have a fight that starts, turn the lights out. That's the only way to stop it.
If it starts tomorrow, you're going to have to move a rock, remove a fish, or watch one die.

A lot better to do your research and ask before adding any fish: list your whole community, including type of corals if any, GIVE YOUR TANK SIZE AND SHAPE [long, deep, wedge, column), and ask, is there going to be trouble adding a [fish name]?