Here at Saltwater Smarts, we frequently advise our fellow salties to introduce fish species to an aquarium in the order of least aggressive to most aggressive.
Using this tactic goes a long way toward ensuring peaceful cohabitation in a fish community for a few reasons: One, it gives the less-assertive specimens a chance to get settled in and establish a territory without the threat of intimidation and bullying. Two, it puts the more aggressive specimens in the status of newcomer, so they’re required to devote their energy to finding a niche in the community rather than tormenting their tankmates.
But what if an elusive species on your wish list becomes available only after you’ve initially stocked your tank and a dominance hierarchy has already been established? Can you add that sought-after fish without throwing your aquarium community into territorial turmoil? The answer to that question may be yes depending on the circumstances.
You might get lucky!
If, based on its relative level of aggressiveness, the new fish in question would have been the last specimen added had it been part of the original livestock lineup, you should be able to go ahead with the introduction without too much concern that it will be bullied. The same may apply to a species that, even if it isn’t particularly assertive compared to the others in the tank, is so different in color, shape, and behavior from the others that they simply don’t view it as a resource competitor.
However, if you have doubts about how the newcomer will be received or are fairly certain that one or more of the fish in the tank will harass it, there are steps you can take to tip the odds in favor of a successful, peaceful introduction.
Tip 1: Rearrange the rockwork
One oft-used technique is to rearrange the rockwork and any other decor in the aquarium right before introducing the new fish. This “reshuffling of the deck” basically erases any established territorial boundaries so all the fish suddenly find themselves with no familiar ground to defend. Essentially, this is the equivalent of moving all the fish, established specimens and the newcomer alike, to an entirely different aquarium.
Tip 2: Restore the right order of introduction
If you know the newcomer is more peaceful by nature than one of the fish in your current livestock lineup, a good option is to re-establish the proper order of introduction—again, from least to most aggressive—by temporarily moving the more aggressive fish to another aquarium and then reintroducing it after the newcomer has had a chance to get settled. The quarantine tank that just held your new fish (for a minimum of four weeks, right?) is the perfect place to hold the prospective bully on a temporary basis.
Tip 3: Segregate the bully
Another alternative is to segregate the bully from the newcomer in the same aquarium by using a tank divider or by placing the bully temporarily in an attached refugium (if your system includes one) or in a suitably sized slotted or perforated container that either floats or hangs on the tank, such as an acrylic acclimation box. The advantage to this approach is that the fish still share the same system water, and therefore can chemically sense, or even see, one another, but they can’t interact physically. In many cases, a truce will continue after the physical barrier has been removed.
No magic bullet!
Keep in mind that these steps will be of little utility if you’re trying to combine species that are essentially incompatible regardless of the order in which they’re introduced. Even if the newcomer and the established fish are compatible on paper, sometimes all measures taken to promote a peaceful introduction fail nonetheless. Be ready to remove and rehome either the victim or the bully if violence ensues.