Brutus and Popeye: Introducing a New Fish into a Hierarchy

Imagine being at home watching tv with the family. Dinner is about to be put on the table when suddenly someone  busts in through the door, telling you that they now live in your home.


This is what happens in an aquarium when we introduce new fish into the community. Their initial reaction is the same as yours, but they don’t have police to call. They deal with it with what they have, and according to the established “pecking order,” or hierarchy in the tank.

Popeye, here’s your new home. Meet Brutus.

In my experience, most fish which go into an aquarium are at a disadvantage. Many are in a weakened and somewhat malnourished state, and have just finished a transit in the past few days which have had them traveling around the world in bags and boxes.

They enter into an aquarium where the community has established an “alpha-fish,” and everyone has fallen into line. The new fish is going to have to find its place both in terms of territory and into the community.

The strongest fish will let them know who is boss very quickly. It will display an aggressive territorial behavior. The weaker fish will defer pretty quickly, as well, some with a territorial fight.

In a perfect world, the new fish would fall right into place in the community. And in that perfect world, Brutus is a big strong fish, and Popeye is a smaller, weaker fish. So when adding a new fish, making sure that you are adding a Popeye into a tank with Brutus increases the chances of quickly establishing the pecking order, because Popeye usually gets the message pretty quickly.

Brutus, here’s your new home. Meet Popeye.

It also works in reverse. Introducing a larger, stronger, healthier fish increases the odds that they will establish their own spot in the hierarchy. While it is not always the case, nor does it usually happen immediately, once Brutus gets his bearings, he usually cleans things up in short order.

The example I usually cite is from a little over a year ago. Someone had a tank which they were unable to add small fish to because the larger fish would harass them and starve them. They were getting discouraged because they wanted some color in the aquarium.

We added a Clarion angel into their community. Clarion angels, unlike many fish, do not have a camouflage mechanism in their coloring. They are bright orange, and generally fend for themselves quite well. Within a few days, he had established himself as the new alpha-fish.

Our customer got the splash of color he wanted, and his aquarium was now complete.

Popeye’s spinach

Fish which have been quarantined and brought up to full health and strength stand a much better chance and usually integrate into the community much more smoothly. A strong fish can usually hold up and repel more of the territorial behaviors of other fish.

There are exceptions: I have personally had experience with three clownfish which had become so large and strong that they claimed the entire aquarium as their territory, and no other fish could be added until they were removed.

This isn’t about fish fighting. It is about them being able to exist peacefully together for a happy aquarium, which is the main goal of an aquarium to begin with.

How does this work for your aquarium?

When adding a fish to your tank, there is always the chance that the fish will not successfully join the community. But by considering two things, you will greatly improve their chances…

Size Matters: Make a definite Brutus and a definite Popeye

When selecting your fish, there should usually be a significant size difference between it and what you already have. By having a distinct size difference from the beginning, you lessen the chance of prolonged or severe territorial behavior and increase the chance of a successful addition to your tank.

Different Shapes: Don’t rile them up unnecessarily

Fish are less tolerant of new fish which are in their families. So selecting fish with different shapes helps to deflect some of the territorial behavior. They don’t necessarily see the new fish as a direct competitor.

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