Aggressive Fish

When it comes to aggressive fish, size does not matter.

Blue Damselfish (Chrysiptera cyanea) and Bicolor Dottybacks (Pictichromis paccagnellae) for example, are relatively small fish, but once they have established their territory in the aquarium, they can be pugnacious and just plain ornery to any newcomer, no matter their size.

Since damsels are relatively hardy fish that are able to cope with typical beginner’s mistakes and poor water quality, they are often sold to the unsuspecting as the ideal beginner’s fish. Good luck to any other fish that are introduced to the aquarium after the aggressive blue damsel!

What Constitutes Fish Aggression And When To Take Action

When a new fish is introduced into a tank with other established fish, a certain amount of chasing is to be expected for a few days. This is where proper quarantining procedure of the new fish serves us well — to make sure that it is not only disease free but also feeding, robust and well-nourished.

Some frayed fins and minor scrapes are to be expected with the occasional chasing, but if aggression is relentless and continues to the point where the fish that is being picked on is injured, hiding in a corner and not feeding, then definite steps should be taken for its removal.

Introduce Non-Aggressive Fish First

As a general rule, non-aggressive fish should always be introduced into the aquarium first. This way, these fish are allowed to establish their territory, swimming space and ‘sleeping quarters’ amongst the live rock. No aggressive fish should be added for at least several weeks or, better yet, not til a couple of months have passed.

When you do eventually introduce more aggressive fish into your tank, they will be newcomers without established territory and will be far more docile.

But this is not to say that these newcomers will not become aggressive and turn on your other fish eventually. Which is why you should always watch for bullying in the form of frequent and unabated chasing once the aggressive fish have established their territory in the tank.

Introducing New Fish If You Already Have Aggressive Fish In Your Tank

If you have already aggressive fish in your tank, introducing healthy, robust and preferably larger fish will give them a better chance of standing up to an aggressor. This is by no means a guarantee, but at least you can be somewhat assured that the odds will be in its favor. All bets are off if your established fish is a blue damsel!

There is another opinion that the newcomer should be put into a floating, transparent betta box so that the other established fish will get ‘used to it’, minimizing aggression once the fish is released into the tank. I can tell you at the outset that this is a bad idea. If anything such a practice only serves to rile up the aggressive fish even more!

At any rate, be prepared to take action to remove a fish if overt aggression does occur.

How To Remove A Fish In Case Of Aggression

The common reasoning is that when aggression occurs, that the newcomer be removed to another tank.

But in our aquariums where there is a lot of rockwork for fish to hide in, this is not always possible. Fish are extremely quick when they want to escape your net. But if yours is a smaller tank of less than 50 gallons, you’ll have an easier job of catching your fish if you remove most of the rockwork.

Often it would be easier to remove the aggressive fish instead of the newcomer. I once saw a friend of mine use a small mirror taped against the glass outside the tank to draw the fish’s attention. Thinking it was a rival fish of the same species, this particular Powder Blue Tang proceeded to attack its own reflection and was too busy to notice the net swooping down from above.

I once used a fish hook and line, baited with a piece of shrimp to remove a Hawaiian Squirellfish (Sargocentron sp.) from my tank. If you are trying this method, remove the barb from the fish hook first by clipping it off so as not to injure the fish too much when you remove the hook from its mouth.

A really good method to remove a fish, provided that the fish is not nocturnal and does not sleep in the rockwork, is to wait a few hours after lights out. Sleeping fish that are out in open water can be easily netted.

Returning The Aggressive Fish To You Main Tank

The question of whether you should return an aggressive fish to the main tank after two weeks in quarantine is a bit of crapshoot. The general idea is that the aggressive fish will be returning to the main display tank as a ‘newcomer’ without territory.

After being away in quarantine for two weeks, most aggressive fish can be returned to the main tank and will likely be model citizens from then on. A few will not, and your only option in this case is to remove the fish once and for all and find alternative housing for it.

Fish Aggression – A Personal Experience

In 2004, when I was a few months into this hobby, my local fish store had a shipment of Royal Grammas (Gramma loreto) from the Caribbean. The Gramma is a relatively rare fish to find in LFS’s in my part of the world, and needless to say I was excited.

I picked out one of the smaller specimens, brought him home and named him ‘Linus’. I was naive about quarantining fish back then so I simply acclimated Linus and added him to my tank.

What I hadn’t counted on was the sheer aggression my Yellowtail Damselfish (Chrysiptera parasema), ‘Bluey’ — yes, I do name my fish — displayed towards the newcomer. Meanwhile, I was frantically on the internet, quickly reading up on what to do in this situation. The news wasn’t what I wanted to hear — either remove the Gramma, or remove the damsel. I had tried removing the Gramma earlier, but Linus moved too swiftly for my net.

After several hours of relentless chasing, Linus found himself in a small crevice under a rock where Bluey could not reach him. I breathed a sigh of relief but soon discovered that the Gramma had developed pop-eye, a drastic swelling of one eye, and a sign that he had injured himself trying to escape from the damselfish.

It was a heartbreaking sight to see that little Royal Gramma, cowering under a rock, with one eye seriously swollen, fearing for its life. Learn from my mistake, and don’t let this happen in your tank. Please.

I managed to remove Linus, he had all but given up trying to stay alive by now, and he let me net him without so much as a struggle. I placed him in a quarantine tank, medicated the water with epsom salts for his pop-eye and then with an antibiotic. Sadly, he died after three days.

Lesson learnt. Aggressive fish are never to be underestimated.

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