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My Aquarium Fish are Fighting! Here is What You Should Do!

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When the peace in my freshwater aquarium was brutally disturbed by my German blue rams chasing each other I did not know what to do. Why were they fighting? Are they going to kill each other? I quickly wanted to learn all the ins and outs, and this is what I found.

Why are aquarium fish fighting or showing aggression towards each other? Fish can fight for many reasons. Most of the times it is to establish a territory. Other reasons are mating, overstocking or a wrong male to female ratio. To prevent fighting, figure out what is wrong and fix it.

Because there can be multiple reasons why your aquarium fish might be showing aggression or even fighting and killing other fish, it is important to go through all the most appearing options.
First, let’s talk about all the reasons why freshwater aquarium fish might be fighting and what makes them aggressive.

What makes aquarium fish aggressive?

With so many things, we first need to find the source of the problem. Up next are the most probable causes for the aggression of your fish. It is important to know these so they do not eventually kill each other.


The first option, and one of the most common reasons, is fish fighting to establish their territory. A lot of fish will fight for a part in the aquarium they call their own. They will most likely not let other fish come close.

Chances are you are the owner of cichlids, with those fish you will definitely see territorial behavior. When they are still new to the tank, all fish will look for their own territory.

Food shortage

When you are feeding your fish daily and providing enough food, this one will most likely not be the reason. If you however do not provide your fish with enough food they will fight over it, and the bigger fish will turn into bullies.

If not all fish in your aquarium can get to the food, they will eventually die.

Mating or Protecting eggs/fry

Taking territorial behavior to the next level, a fish couple will do anything to protect their eggs and fry. At first, they will look for the perfect spot to spawn. When they have found it, they will allow no other fish near it.

If you have a pair of fish in your aquarium chances are high that you will see them chasing away other fish. If your aquarium is big enough and there are enough spots to hide, this form of aggression will be harmless.

Some examples of fish that will do this are angel fish, German blue rams, kribensis fish and many more cichlids. Make sure to research your fish if you are in doubt.

Overstocked or understocked

When your aquarium has way too many fish in them, they will struggle to find their own territory. This might mean that they are all constantly fighting to establish one. Make sure that your aquarium provides enough space for the fish you are housing, and add objects that break line of sight.

Using rocks, plants and wood to break the line of sight is an effective way to give your fish a break. They do not see each other constantly.

Your tank might also be understocked. For example, schooling fish have to live in a school to disperse aggression. The perfect example fish would be a tiger barb. This fish is super popular to keep but a notorious fin nipper.

If you keep your barbs with numbers below 10 at a time, they will show more aggression compared to larger numbers. The same goes for pea puffers, they can live along or in a group. Just two of them would be a problem.

Male Female Ratio

Loads of people have kept guppies at some point. If you would add 1 female and 3 males in your aquarium, the males will harass the female till she dies. This is extremely painful to watch and just not the way to go.

Instead, they require a ratio of around 1 male to 3 females. This way, the male will divide its attention and will not harass the females as much. This is crucial to keep a peaceful aquarium.

Furthermore, for various fish species you can not keep 2 male fish together. They will fight to establish dominance, and in a lot of situations they will not stop until there is one left.

I have experienced this myself with my German blue rams, I accidentally bought two males instead of a couple. Luckily I could swap one of the males for a female at my local fish store.

Again, always research the fish you are getting.

How to spot aggression in fish

The key to maintaining a peaceful aquarium is spotting aggression early on. Doing so will allow you to make the necessary changes before it is too late and your fish might even kill each other.

Signs/symptoms of bullying and fighting

When you see that your fish are fighting, it is really obvious. They will aggressively chase each other, bite at each other’s fins and even lock in mouths.

Fighting can be to establish dominance, but it can escalate to straight bullying. Bigger/stronger fish will often bully smaller/weaker fish. You will notice who is the bully and who is the victim when you see it.

When you are not there to see them fighting, signs/symptoms will be damaged fins, missing scales or even missing eyes. These wounds are extremely susceptible to fungal/bacterial infections and should be treated accordingly.

Results of aggression in a tank is fish that are deprived of food and shelter. They will be in a really bad condition after a while.

Make sure it is not mating behavior

As a small side note, mating behavior can look a lot light fighting. I have told you about male guppies harassing female guppies. And a lot of other fish species will chase each other around the aquarium.

To tell the two apart, look at the physical state of the victim fish. If it is extremely timid, is hiding in a corner (or even at the top) of the tank and is barely eating, it probably is fighting.

Ways to prevent aggression

So now, the most important part: how to prevent the aggression. What can you do to make your fish stop fighting.

Adding fish

When there is just one fish of a given species that harasses other fish in the tank, adding two or three of that species can help as a distraction. They will turn their attention to their own kind.

This is where adding a fish might be the solution. Also for the tiger barb/schooling fish example. Fish in bigger numbers will display less aggression.

Removing fish

When you see an obvious bully in your tank, giving that fish a “time out” in another separate aquarium has proven to work for a lot of people. Do not ask me why on a science basis, but it works. Remove it for a couple of days to a separate tank and introduce it again.

If you are housing a group of fish, for example a bigger cichlid tank, you might also see an obvious victim. When all the other fish are turning against one weaker fish, remove the victim for its own safety. Try to find it another tank, whether this is with another enthusiast or back to your local fish store.

We have talked about a variety of reasons for fish to fight, but just like humans, sometimes there is no reason. Some fish will just not like each other. If this is the case, remove one of the two parties is your best option.

Rearrange aquarium

When you find your fish fighting over a territory, first of all make sure your tank is not overstocked. If this is not the case, rearranging your aquarium might just do the trick.

The fish will not know the borders of its territory and it might cool down.

Research fish species

One of the best ways to prevent aggression is to research the fish species you (plan to) add to your tank. While we are all looking for a beautiful aquarium, not all fish species go well together.

Truthful store employees will tell you what will go well and what does not. But experience from a lot of aquarium enthusiasts have proven over and over that not all store owners/employees are knowledgeable. Therefore it is best to do your own research. Ask around and see who has experience with the fish you are looking into.