When Should Squabbling Marine Fish Be Separated?

Fish that share similarities are more likely to fight when confined to a smaller space

Fish that share similarities are more likely to fight when confined to a smaller space


I recently introduced a Kole tang to my tank, and it keeps getting in fights with the resident yellow tang. So far, neither one has been seriously hurt, but I’m worried about where this is heading. Do I need to separate these two, or will they eventually chill out and coexist peacefully?”

Over the years, I’ve often been asked questions like this or read similar ones in print media and online forums. When a compatibility issue arises between fish—as is especially common when a new specimen is introduced to an established community—the combatants may eventually sort things out and establish a wary truce or there may be no alternative to removing one or the other.

Here are some conditions that, in my opinion, necessitate separating the warring factions:

There’s no room for escape

Remember, confined spaces tend to magnify aggression. If the tank is too small to allow the subordinate specimen to flee the aggressor’s immediate vicinity or the aquascaping doesn’t provide an adequate number of retreats and hidey holes, the two won’t be able to stay out of each other’s way and will likely continue fighting.

The aggression goes beyond bluster

If the aggressive behavior greatly exceeds mere displaying and posturing or passive-aggressiveness (“What? Who, me? I’m not trying to exert my dominance or anything! I just happen to be grazing on the substrate just outside the little cave you’re hiding in with my body pressed up against the opening!”) to include full-on attacks, separation is probably in order.

If you don’t actually catch the bully in the act (these hostile interactions can be fleeting), injuries on the victim—notched or torn fins, missing scales, etc.—are another telltale sign to watch for.

One possesses dangerous weaponry

In this case, one of the combatants is equipped with some type of defensive mechanism, such as sharp teeth or venomous spines, that can inflict major injury on the other with little effort. This can apply to either the aggressor or the aggressee.

One is excessively stressed

A situation in which one party constantly hides in the rockwork, cowers in a corner, refuses to come out to feed, continuously exhibits stress coloration, or otherwise shows signs of excessive stress also indicates the need for separation. Otherwise, the stressed-out fish may very well develop a disease and die or leap from the tank with the same outcome.

The two are just too similar

Fish that are very similar in coloration, shape, feeding behavior, or the niche they occupy on the reef are much more likely to fight incessantly when confined together in the same tank than dissimilar species are. So, if you end up with two specimens that are just too alike to like each other, separation may be your only option.

Photo credit: Nathan Rupert

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About Jeff Kurtz

Jeff Kurtz is the Co-founder/Editor of Saltwater Smarts, former Senior Consulting Editor for Tropical Fish Hobbyist Magazine, and the aquarist formerly known as “The Salt Creep.” He has been an aquarium hobbyist for over 30 years and is an avid scuba diver.

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