HELP! My Fish Are Showing Aggression Towards Each Other

Although having an aquarium with numerous fish of different sizes and species makes a beautiful and exciting tank, we need to be responsible aquarists and make sure that we do our research before just adding fish to our aquariums that we think look cool at our LFS. There are many great ways to research before purchasing and even some tricks that we can use in the event that we still have fish that display aggressive behavior after their introduction to our tanks. Our goal for these tips and tricks is that they will help you make better choices when stocking your tank, reduce stress on new as well as existing fish, and above all, keep more fish alive!

First, A Little Bit About Fish Biology

Fish, by nature, are competitive creatures. They display much of the same aggression with regards to feeding and territoriality with the ultimate goal of reproduction that is found throughout the animal world. Our tanks serve as a micro version of the larger habitat these marine fish species usually reside in. This means that careful planning should be used when you are going to stock your tank with fish and invertebrates. Taking into account not only what a species will contribute to the environment, but also how will this individual play a role in the greater “social fish structure,” are both important things to consider before making a purchase.

Fish, in particular, tend to show aggression towards each other and other fish.

img_2897Fish are herbivores. More herbivores in the tank mean less algae that can be consumed for food. Fish have evolved to become territorial over their patch of algae in fear of losing their food source if another fish takes over. It’s not just a matter of being selfish – it’s life or death. There are very few other herbivores that fish tolerate.

Aquariums are the most common place for fish to exhibit aggression, due to tanks’ small size and cramped quarters, causing the fish stress. Despite the efforts of even the most conscientious aquarists, it is hard for the size of an aquarium to compare to the environment tangs are used to in their natural environment. This is especially true for fish that are a member of the Acanthuridae family, which include power blue tangs and Achilles tangs, that swim several miles a day in the wild. In cramped aquariums without the proper space to exercise, they start to get stressed and respond by acting aggressively.

Who Will My Fish Be Most Aggressive With?

Although fish can be aggressive with other types of fish, they are most aggressive with members of the same genus and species as their own. Competition for food fosters the aggression between similar members. Fish are more likely to fight with other fish of the same species since they have identical diets to their own. Fish are even known to stand guard for a particular patch of rock when they are not schooling with their group.

Tips and Tricks for Keeping Multiple Fish of the Same Species/Shape

Although there is not a single foolproof solution to bring peace to your riled up fish, there are some approaches you can take in an attempt to calm everyone down.

  1. Keep food plentiful. Since one of the main causes of competition is food, keeping the fish well fed helps. Of course over feeding is never a good thing, but placing enough food in the tank so that it is gone by the end of the day but available throughout most of the day, will decrease the amount of competition over food.
  1. Re-arrange your aquarium rocks. Mixing up your aquarium scene temporarily confuses your fish, making them think they are no longer in their territory. Although this might seem cruel, it causes no harm to the fish and without their familiar territory to defend, aggressive behavior towards others may decrease.
  1. Acquire some mirrors. Taping a mirror to the inside of your tank is a cheap and easy method to try to lessen competition between tangs. The use of a mirror provides a harmless, reflection friend for the fish to compete. This way, instead of fighting with the other fish, the fish can fight “itself.” Although cheap and easy, this method has had mixed success results.
    You can see the reflection of the tank in the mirror.

    You can see the reflection of the tank in the mirror.  This is what the fish see and what they will fight against instead of each other.

    The bottom right corner shows the mirror tile placed in the corner of the tank.

    The bottom right corner shows the mirror tile placed in the corner of the tank. Mirrors can be used to decrease aggression between fish.


  1. Removing and re-introducing the problem fish. You might think that this method seems pointless, but what removing a fish and reintroducing it later does is disturb the pecking order. He may be the big cheese when you remove him, but after a couple days or weeks, new roles will be assumed in the tank and the former bully will no longer have his role.

Introducing New Fish To Your Tank

Introducing new fish into your tank can be tricky. Here are some trips to smooth the process.

Never introduce new fish alone!

Never introduce new fish alone!

  1. Introducing multiple fish at once – NEVER just one by itself. Just like a kid entering a new school, making friends can be difficult by yourself. The odds of three established fish against one newcomer are not good. It may end up working out in the end, but the newbie will be stressed beyond belief. Even if the odds were two established fish to one new fish, or even one to one, difficulties could ensue. You have to consider the health and condition of the fish on either side. The established fish will be fat, healthy, and thriving, whereas the new additions might not be in the best of health after traveling all the way to get to you. They may be thinner due to sparse food during travel, have a weakened immune system, and already be tense.* The more fish you introduce at once, the more of a distraction will be present, and the less of a chance that one individual fish will be targeted.

*NOTE: Aquarists might want to consider quarantining any new fish for a few weeks to fatten them up and check for diseases. This way, when they are introduced to the tank, their health isn’t as compromised as they were when they arrived and they have a better chance of standing their own.

  1. Think about the size of the fish you are introducing. You might think that adding fish of similar size to the ones in the tank might be the move. If all the fish are relatively the same size, everyone will get along, right? This may not always be the case. Adding fish of similar size creates more competition for the pecking order because there is no size variation to work the order out naturally. Adding larger fish than the fish in the tank might intimidate them, and cause them to act less aggressively. Adding smaller fish might also be beneficial because the larger fish in the tank would see them as less of a threat, and would therefore leave them alone. This method isn’t foolproof, however, because some fish may not get as intimidated as others, making the size of the fish you introduce a moot point.

An Ounce of Prevention and Research Goes A Long Way

Before you go out and outfit your aquarium with new fish, do your research! Don’t buy the first brightly colored, fast swimming fish you see. Take some time to plan your purchases out beforehand. Make sure you do some research online to tap into the wealth of information available. A smart aquarist is a happy aquarist!

Summary: It May Take Some Time For Your Fish To Get Along

Although it may take a little bit of time, it IS possible to make your fish get along. Aquarists may have to use a combination of the tricks to get the fish to mesh well. It is definitely not a one and done deal. Some tricks may work better than others. Find out what works for your aquarium and its contents. In addition to the tips, be smart about your purchases. Do what you can before hitting the stores or Internet.



Image Sources:

First Photo: Ally Kontra

Tang Mirror Photos: Brian Dunat

Never introduce new fish alone: