Life After Purchase: A Guide to Introducing New Fish to Your Aquarium’s Social Dynamics

Moving can be tough on a kid. New places, new school, making new friends, meeting new bullies . . . I’m sure some of you can relate. So can your fish. Life can be tough even after getting shipped halfway around the world. So how can we, as responsible aquarists, help them out? The answer can be complicated, as social dynamics in an aquarium can be complex and ever-changing!

That yellow tang that you bought six months ago is hitting maturity and is starting to stake out a larger territory, pushing that algae blenny competing for the tang’s favorite grazing rocks toward the clownfish pair in the corner who, just by sheer chance, are feeling frisky for the one-hundredth time this year and protecting their spawning area.

The pajama cardinals that usually hang out near the clownfish have migrated passed the flame hawk (who is contemplating whether the new cleaner shrimp you bought last week will fit in its mouth) and have settled underneath the royal gramma’s shelf so she now feels she has to show how big and bad she is to these new intruders.

The dominant female lyretail anthias begins her plans to take over the harem and oust the male that has been getting a little skinny avoiding the yellow tang at feeding time.

But wait! The provider is coming! They all stop for a few minutes when you walk up to the tank to feed and contemplate adding a new fish because “everyone is getting along so great right now.”

These dynamics must be observed by the aquarist daily. Noticing trends, adjusting rockwork, target feeding someone a bit more—all can alleviate some of the growing pains that a tank can have. Obviously, the fish that are already in there have a much better chance of survival than some newcomer that is fresh off the plane and just “thrown to the wolves.”

Social dynamics are ever changing, especially when multiples of the same species are kept together.

Social dynamics are ever changing, especially when multiples of the same species are kept together.

So how does one add a new fish intelligently? Here are some tips and tricks that may help the process. Clearly this is not a shortcut to researching the individual fishes you intend to keep and determining whether they are appropriate tankmates for your current inhabitants, but it can make a substantial difference.

Quarantine and condition

Whether it is a passive quarantine (observation without medicating) or proactive quarantine (prevention of disease through medication), conditioning and quarantining a new fish is the number one way to ensure things go smoothly when introducing it. Fish that are physically in good condition and are not suffering from any health maladies are better able to avoid and defend themselves from fish already established in the tank. Quarantine allows fish to rest after their big move as well as settle in to your time schedule and feeding regimen.

I recommend a quarantine period of at least two weeks. Basic, bare-bones quarantine systems should be an appropriately sized aquarium for the type of fish you are quarantining—usually between 20 and 60 gallons for the average hobbyist. An aquarium top, filter, heater, and PVC pipes for cover are all you need to make a functional quarantine system. The quarantine system should be placed at least 20 feet away from the display to prevent cross contamination and preferably should cycled before adding fish.

Separation makes the heart grow fonder

It amazes me that so many aquarists just cross their fingers when adding new fish to their aquarium. Sure, it goes alright a lot of the time, but it can go horribly wrong too. Social acclimation is a tool that should be utilized more than it is. When public aquariums move fish into predatory shark tanks, they often utilize an isolation pool. This isolation pool is usually separated from the main tank with a plastic mesh gate. The new fish are able to orient themselves, de-stress, and acclimate to tank conditions without harassment from established fish in the system before being released. This can be imitated in an aquarium very easily. You can buy premade acclimation boxes or make your own with a large plastic container with premade holes. You can also use a soldering iron to make holes in the plastic for ventilation.

An example of a plastic container that can possibly be modified into a social-acclimation box.

An example of a plastic container that can possibly be modified into a social-acclimation box.

Larger fish obviously would need more space than a plastic container can provide. This can be remedied with egg crate cut to fit inside the tank and vinyl tubing cut lengthwise and fitted on each end of the egg crate to create a snug seal from front to back in your aquarium. This divides a section of the tank for your new inhabitants to acclimate.

Separation acclimation allows established fish to see, smell, and interact with the new fish without physical damage to either animal.

A premade social-acclimation box by Seaside Aquatics holding a shy blue-spotted jawfish.

A premade social-acclimation box by Seaside Aquatics holding a shy blue-spotted jawfish.

Lights ON, not OFF!

When finally releasing your new fish, do so with subdued lighting, but I would recommend (contrary to popular belief) not doing it right before the lights go out so the new fish can be introduced to the established tankmates. I’ve seen the “lights-out” approach to acclimation go horribly wrong, and because the tank is dark, you are unable to assess how the new fish is doing or to take action if needed.

The new fish is also at a significantly greater disadvantage. It has never explored the caves in your live rock and has no idea where your other fish sleep or have staked out a territory. I’ve seen new fish get pummeled all night because they picked the wrong sleeping spot but are obviously too disoriented to find their way out of that sleeping spot! The first day is an “intelligence-gathering mission” for your new fish, and it is a lot easier to gather intelligence when you can see!

Break up the friction

The first day a new fish is released, it is important to assess how it is doing. If things are going badly, don’t hesitate to take action. Something as simple as breaking up territories by carefully rearranging a few rocks can help significantly. Mirrors taped to the side of the tank can also confuse territorial fish into attacking their reflection versus the newcomer for awhile. Increasing your feedings for the first few days can also distract fish into accepting their new tankmate.

Just sump it

Persistently aggressive fish can be a major issue. In a perfect world, these fish would be added last to the aquarium after everyone else has settled in and staked a claim to their homes. You can simulate this order by temporarily removing a persistently aggressive fish and putting it into a sump or your quarantine tank (after it’s been sterilized properly), then reintroducing it to the tank after a week or two.

The ever-changing dynamics of an aquarium can be a daunting challenge, so be prepared. With these steps, you can greatly improve your chances of introducing fish into your aquarium without aggression or mitigate those aggression issues. Remember that all fish are different and sometimes even the best-laid plans can still go off track.


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About Paul Poeschl

Paul Poeschl is currently employed in public aquaria. Previously he was supervisor of collections for Living Exhibits Inc. where he helped manage, collect, and acquire fish for exhibits in zoos and aquariums across the country. He also spent several years as the lead fish husbandry technician at LiveAquaria's Diver's Den. Paul has been in the industry for over a decade and a hobbyist for twenty five years.

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