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Why do fish behave aggressively? 5 important Reasons

Updated: Feb 1, 2023

Why fish are aggressive towards each other? This is the most common question asked by a beginner aquarist. It is mostly seen that an aquarist chooses his fish without any guidance from the seller. An aquarist especially a beginner is so taken up by the vivid colors of the fish that he or she does not realize which species is bringing at home. This happens most of the time to an aquarist only to find that there is a conflict of species. We all have done a mistake in our hobby sometime or another. I remember a hobbyist coming to our outlet and cribbing about a seller who sold him Angel fish and guppies together for his new tank. Don’t know who is to be blamed for such a mistake. The seller did not guide the buyer nor did the buyer ask the question of “COMPATIBILITY” there are other reasons too for aggressiveness which we will look into in this article. Always Ask the first question of whether selected species are compatible otherwise you will land up with a monster in a tank.

Community fish will not attack each other as long as the fish brought is of the same size and temperament.

There are different types of fish species that you can have in the tank. My favorite was the African cichlid tank. Before we dive into the species category let's talk about sudden aggressiveness in a fish. There are reasons as to why suddenly you will find aggressiveness in the fish.

There are many reasons why the fish behaves abnormally especially during feeding and breeding time.

Feeding: The dominant fish will try to bully other slow-moving fish during the feeding time and eat aggressively. The fast-moving fish will outdo and eat more aggressively than other slow-moving fish. They will even try to snatch the meal from another fish's mouth. This is normal behavior. The best thing to do is to spread the food across the tank rather than in a single place.

Mix species: This is the most occurring problem in a community tank. Most of the time the hobbyist brings a monster to his tank. He gets carried away at the fish shop due to its vibrant colors, for example, jack Dempsey when it is juvenile it looks so beautiful compelling the hobbyist to get it and put it into his aquarium having community fish. This will make a living hell for the inhabitants of the tank. I have seen people putting guppies with Angels too. Resulting in guppies losing their longer and wider caudal fin. It's sad to see this in an aquarium. If the tank has its species tank it's far better than the tetra tanks or Goldfish only tank. Proper research needs to be done while selecting the fish compatibility. It is no point in introducing a fish just because it looks beautiful in the fish shop. Looks can be deceiving.

Angelfish & agression
Angel fish breeding & Agression

Breeding: This I have experienced firsthand in a tank having Angels. When the Angels are forming a pair the male Angel will not tolerate even their own coming towards his selected potential female Angelfish. He fearlessly defends her and his territory. I have seen the male chasing in a planted tank all other species too. This becomes a painful experience for other species as their movement is restricted which can lead to stress. More so ever when the aquarium is small and the pairs are formed. The best thing is to put the pairs in another tank. It is also seen that if there is an aggressive fish in a tank, it will just stress the female fish which is laying eggs. A breeding betta male will chase its betta female. The female needs to be separated at once the eggs have been laid. Otherwise, the male will stress the female, and eventually, it will die. Even the jewel cichlids will defend their territory during breeding. This is all a natural behavior. The best thing is to provide hiding places for the females and pairs. I have observed that The bigger the tank the lesser the aggression during the breeding process.

Aggressive Malawi Cichlids
Aggressive Malawi Cichlids

Dominance hierarchy: There is always a “BOSS” in a tank who likes to put the order in the tank. He will be a male dominating all others in a tank. He will show aggressive behavior. He is like the principal of the school where the students are mostly afraid to cross paths, Right? The male fish will flaunt all its colors. This behavior I have seen in Malawi Cichlids. It is easily identified in an African Cichlid tank. If you observe there will be one particular male dominant cichlid who will set the house in order. He will chase the fish which does not fall in its line. It does not remain the same always. The “ BOSS” of the tank when weakens another one takes over. Suddenly the previous Boss loses their place in the hierarchy. I have also observed in an African cichlid tank that even if there are other males present in the Aquarium they will not show their actual colors because of the behavior of the dominant male. Once the dominant male loses his place in the hierarchy they start showing his colors and will go all out to be up in the hierarchy.

Introduction of New Fish: A New fish is always chased in an Aquarium. A New fish when introduced will be at some corner of the tank. Provide some hiding place in an Aquarium. Turn off the lights after the introduction of the new fish. Let it rest. This is normal behavior.

Oscar fish
Oscar (Monster fish) beginner should avoid

Size of the tank: I had this client who had 2.5 feet in length aquarium with Oscar, sevrams, and parrot fish. When they were small they looked for the right quantity for the tank. The Oscar became quite big and started forming a pair, and so has the parrot fish. Oscar found the space too small so as the other fish. This lead to stress. The size of the tank does lead to aggressiveness at a later stage.

Arowana monster fish
Arowana (Monster fish category)

So how can you curtail the aggressive behavior?

My favorite technique is to change the position of the Ornamental décor in the tank and place it in different places. This normally confuses the fish as it needs to re-establish its territory.

It becomes necessary to isolate the fish in another tank for a few days.

  • Beginners should avoid monster fish like Arowana, Flowerhorn, Oscar, and other aggressive cichlids in a community tank. Avoid Barbs in a community tank. There are some Gourami fish which are considered fin nippers. Or a shark with a red tail. just have this as a single species. They can be aggressive towards each other.

  • You can put the semi-Aggressive fish together. They will keep on chasing each other. No single fish will be targeted and chased by all. Quite often it works.

  • Feed them well and dont crowd an Aquarium.

  • Sometimes if nothing works, approach your local fish shop and request them to take back the aggressive fish in exchange for another fish.

  • Research your fish and check for its compatibility with your existing fish in an aquarium, Better safe than be sorry.

Please find once again compatibility pdf file for your understanding. Download the file to understand behaviour pattern in Fish.

Freshwater-Fish-Compatibility-Chart (1)
Download PDF • 1.41MB

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Author :- Sunil Dcosta for

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