How to Manage an Aggressive Fish

Aggression can begin at any time...

Once you have established your aquarium and have a nice, congenial mix of fish, you may one day find yourself alarmed and dismayed when one fish suddenly takes a notion to start terrorizing one or more of it’s tank-mates. This can happen for a number of reasons.

Sometimes Aggressive Fish are Happier Alone

 Title: Lionfish in the Red Sea ~ Attribution License: http://creativecommons.org/licenses/by/2.5/ ~Photographer: Zpyder: everystockphoto.com
Title: Lionfish in the Red Sea ~ Attribution License: http://creativecommons.org/licenses/by/2.5/ ~Photographer: Zpyder: everystockphoto.com
  1. One fish may become ill causing others to begin picking on it.
  2. Some fish become aggressive when they reach a certain point of maturity. This is one of the many reasons to do your research before you choose your fish.
  3. An increase in temperature or change of seasons may cause some fish to become aggressive.
  4. Some fish become aggressive when they are ill.


Regardless of the reason, you will want to remove the aggressor. Even if the fish being picked on is being picked on due to illness, you will want to separate the aggressor. Once they start aggressing, they usually continue. If you remove the object of aggression, the aggressive fish will be highly likely to pick on someone else. The exception to this would be if many fish are picking on one fish. That would be a clear indication that the one being picked on is ill and should be removed.

If you do determine that the aggression is occurring either because the aggressor is ill or because the fish that is being picked on is ill, you will want to treat for illness in both the main tank and the tank you move the aggressor or ill fish to. The first treatment should be Melafix, a natural, overall tonic that has calming properties. Keep this on hand so that you can use it as needed to handle illness and stress. If the problem isn’t cured with Melafix, note the symptoms of any illness your fish are exhibiting and take a trip to your local fish store to locate the proper medication. Whatever, the illness, Melafix may cure it, and if it doesn’t, it will calm and soothe without doing any harm.

Now, back to your aggressive fish. Where will you put it? It is always a good idea to have a small tank ready to receive injured, ill or aggressive fish. One good way to keep a tank set up and ready is to keep a three, five, or ten gallon tank set up with a single Siamese fighting fish. In this way, you will always have a seasoned tank ready to act as a spare if needed. When you need to use this tank temporarily to house a sick or aggressive fish, you can always move the beta out to a fishbowl temporarily.

Once you have separated your aggressive fish, you will need to decide what to do with it in the long term. I am very fortunate to have a local fish store that will trade fish, so I have taken quite a few fish that got too big or aggressive back to my local fish store and traded for something more appropriate. Another option, of course is to advertise in your local paper or on Craig’s List to sell or trade your aggressive fish.

If you are very fond of the fish, you may decide to set it up in it’s own tank. I have done this a number of times with favorite aggressors! The advantage of keeping this kind of fish in it’s own tank is that aggressive fish are usually full of personality. Kept on their own or with one or two fish that agree with them, they can be very special pets.

Copyright: SuzanneBennett: June 6, 2009

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Comments 6 comments

Peggy W profile image

Peggy W 3 years ago from Houston, Texas

We no longer have fish tanks but I found this informative none-the-less. You are fortunate to have a fish store that will let you trade fish if they get too big or aggressive for the tank. Up and interesting votes.


justmesuzanne profile image

justmesuzanne 3 years ago from Texas Author

Many thanks! Yes, I no longer have fish except for a small pond with mosquito fish in my back yard. At one time, I had 17 tanks! :D I learned all I could, and now I am done! :)


Peggy W profile image

Peggy W 3 years ago from Houston, Texas

17 tanks! Wow! I only had 2 at one point in our lives. Going to pin this and also share.


Jeannieinabottle profile image

Jeannieinabottle 3 years ago from Baltimore, MD

My parents used to have two fish tanks. I remember coming home from school when I was little and finding my mother had moved a fish or two from one tank to another while I was out. She would tell me she had to separate two fish that did not get along or remove a "problem fish" from one tank. It was kind of funny, but it makes sense now. Great hub!


rose-the planner profile image

rose-the planner 3 years ago from Toronto, Ontario-Canada

I find this subject so interesting. I found your tips on dealing with aggressive fish very informative. Occasionally I visit this very large aquarium superstore that is near my house because it is simply amazing in there and the various types of fish are incredible. I notice that they clearly label which fish to not put together because some are more aggressive than others. There really are many things you need to know in order to maintain an aquarium. This was a great article. Thank you for sharing. (Voted Up) -Rose


justmesuzanne profile image

justmesuzanne 3 years ago from Texas Author

Thanks, Peggy!

Yes, Jeannie, there really is a lot to know about fish of all kinds. Many people have terrible experiences with aquariums because they mix fish indiscriminately.

Rose, it's good that your LFS (local fish store) labels their fish well. It is still wise for the purchaser to do some research first. Very often the labels are wrong and the employees are clueless! :D

Thanks for all the kind comments, all! :)

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