Are there 'aggressive' or 'community' fish in saltwater? FYI

[Sk8r]

Not in the sense freshwater hobby designates them.
Fish that eat other fish are best not housed with fish that will fit in their mouths. Lionfish, eg.
That leaves us with fish that prey on crustaceans and krill and such. Or algae.
But in very many other cases tank size can start a fish-war even among these relatively easygoing types. And fish do come equipped with an attitude that fits their adult size.
Bear in mind that blennies and gobies and dwarf angels, etc. are sold as adults, but many fish are sold as juvies, because---think about it---selling them as adults would require a monster bag for takehome. And juvies will grow to adult size no matter the size of your tank. Tank size does not limit their growth. If the tank is too small you will have a 'failure to thrive' that may involve the fish it harasses, as well. A fish that's outgrowing its space may well take after its tank mates. It just doesn't end well.
So when planning your tank, plan for adequate space. Don't buy fish for the tank you hope someday to have, because things start to go wrong long before that happens.

 

[hkgar]

There will be territorial conflict and also food fights. When I feed Romaine to my Alantic (9") Blue Tang and Two Bar Rabbitfish the Tang does nip a bit at the Rabbit but nothing serious. My Swallowtail Angel will chase the Bellus.

A group of Chromis will almost always end up with one winner.

 

[Sk8r]

There are a few that drive 'territory' to new levels: dottybacks and many damsels, in particular.

 

[Timfish]

And let's not forget sexual maturity. Tangs are a good example. Most tangs are juveniles when sold and aren't mature until they're 5-7 years old. Several tangs that cohabitate as juviniels may not when they become mature. Even in big tanks of several hundred gallons I've seen a dominate tang in a greoup that appeared to be stable for years start taking out lesser ranking tangs. Over the years I've personally seen a Maroon clown and a Coral Beauty angel become dominate and systematicly kill off the other fish in their tank.

 

[Sk8r]

I had a humbug damsel that made about 8" --- future size does affect aggression, and she tolerated everybody she grew up with---except chromis: damsels hate chromis---but when I lost some fish in a winter power-out, I couldn't bring replacements in. She wouldn't tolerate them. She was a cool fish. She hummed when annoyed, loudly enough to hear in the next room.

Rabbits turn killer when they hit a certain size and start feeling cramped. They do freakouts if they feel cornered, rather than aggression---and use that venomous spine on whatever fish has been unlucky enough to box the rabbit into a corner.

 

[hkgar]

My Rabbit raises his fins all the time with the Atlantic Blue when they are quibbling over the Romaine lettuce. Other times they can be found swimming around side by side. Seems to be a love/hate relationship