Fish killing in the tank?!

This is a really big problem that a lot of tank owners face. It is not something that any of us want to have a problem with, however. Having fish killing other fish, well to fix it you will first need to understand why they are doing it. So let’s start with talking about why fish are aggressive so that you can better solve this problem.

What makes fish so aggressive?

There are many factors that determine whether or not a fish will be aggressive and you will have a fish killing on your hands. There are fish that are known as social fish, these can even become aggressive. They do not generally have territories, but they will fight for positions in a group. Which is why you will see a bit of chasing and aggression with your fish. The larger that the group is the and if it contains a decent balance of both females and males, then you should not see any serious damage done.

A fish that is known for it’s a good example of a territorial fish is the cichlid. It uses territories for mating displays for a female, raising children, or, of course, for food. The problems generally start when there is not enough space for the fish to hold territories. This is what can start the fish killing. It may only start with aggressive behavior, or harassment, or damage, but can end in a fish killing. Now let’s discuss how you can identify a fish’s behavior when it starts to show aggression.

How to notice aggression with your fish

Anyone who chooses to own an aquarium needs to be aware of the signs of aggression in fish. You should also know that these issues need to be stopped, specifically if you are dealing with fish killing. The one starting the mess usually starts this by flaring its gills or fins. The displays are usually meant for giving the other fish a chance to get out of this fishes way.

If this is not taken in, or due to the fact that this is an aquarium, you may face an issue as the fish is not able to get far enough away from the aggressive fish. This then makes the fish react as if it was challenged and will chase or attack the fish, depending on if the attacker is weaker or not; The attacked fish will be near the top of the tank or in the back of the filter. It will be breathing heavily, and showing dark colors or muted colors.

fish killingIf this doesn’t get fixed, the weaker fish may end up injured, with torn fins, damage to mouth or eyes. Worst case scenario, you may end up with a fish killing another. With injuries, the fish can succumb to their immune system weakening. This can cause a fish to stop eating.  This is something that you definitely want to avoid. You may see injuries like the one featured if you aren’t careful.

Create a school?

If the fish that you are dealing with is a schooling fish, one that generally lives with a larger group of it’s kind, then adding more of that species can help. This will not work every time, bit it may be what your fish is acting out because of. There are a lot of reasons that fish act out, the tiger barb fish are a well-known fish that will act out in small groups. If you own less than 10 of this fish you may witness a good bit of biting and fighting or causing a fish killing. It is also really important to note what the ratio of males to females. This can also cause you to have problems amongst your fish.

Removing the fish

If the aggressive fish is not letting up and you have tried everything else, such as rearranging the tank, time-outs, and other methods. Then it may be time to remove a fish. You can choose to remove the aggressive fish, and it can remove the problem. If the weaker fish is being bullied by other fish as well, you may opt to remove it as well to save it. The problem may be that there are too many fish in the aggressor’s mind. Removing this fish may fix it.


You can also choose to stock so that the density is higher making it hard for any fish to claim a territory that would be stable. This will allow the fishes aggression to eventually go away. This often works well with cichlid fish, as well as damselfish, mudskippers, and several others. The only negative for this is that it raises the amount of maintenance and the filtration that is required for quality water.


There are no guarantees for any of these solutions, which is why you really need to research any fish you want to place in your aquarium.