Are My Fish Eating Each Other?

Fish In A Tank

This question from a reader has reached a sensitive point for me, because I too have experienced what is happening to her, aquarium fish eating other fish. This has probably happened to many of you.

“I have two Ancistrus in a mixed tropical aquarium. I have noticed in the last few weeks that fish have started to disappear, and I have attributed these disappearances to my red-tailed sharks, but tonight I found a fish, initially perfectly healthy, dead. The fish was stuck under the bridge that is the home of my adult male Ancistrus and was already half-eaten. I wonder if he is the culprit behind the other disappearances. Has anyone noticed this cannibalistic behavior?”

Fish are opportunists

I’ve seen this story quite often: A fish owner observes the disappearance of some fish, and then sees a fish normally peaceful, eating from the corpse of a fish that has disappeared.

The immediate assumption is that the peaceful fish is actually the one with cannibalistic behavior.

I thought about the same thing when it first happened to me years ago. After seeing the body of one of my favorite fish being eaten, I was so enraged at the “guilty” fish.

I decided to consult with someone wiser than myself before doing anything irrational and learned a valuable lesson. He told me that seeing one fish eating another means nothing.

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Fish in the aquarium will quickly take advantage of the situation in which a fish dies. Even a fish that is still alive, but is very weak or sick, will be chosen by its aquarium colleagues and eaten.

Why do fish eat each other?

Fish are opportunistic, as are other creatures. If the food is presented in any form, they will eat it.

Sure, maybe the food was your best swimming friend the day before, but today is lunch. Everything is part of the circle of life.

So how does a fish owner know who or which one is the real culprit? Sometimes it can be an obvious bully. Other times it might be a peaceful fish, but it’s just as likely that the culprit isn’t another fish.

A series of deaths could be the result of a change in water conditions, an undetectable disease outbreak, or simply stress.

The key is to carefully observe what is happening in the aquarium. This is especially true if you have added a new fish or whenever something changes in the aquarium.

Keep an eye on the habitat and find out what the water conditions are to avoid invisible changes that can trigger the loss of fish.

New fish

Aquarium FishNew fish in an aquarium are closely examined by the other fish. A practice of pinching exists even in an aquarium of peaceful fish.

The territories have already been established and all eyes are on the newcomers to see what space they will choose.

If other fish feel threatened in any way, they can start fighting. This can happen even in the case of normally peaceful fish.

Observe the fish closely each time you add a new one to the aquarium. If you notice signs of aggression, use the old trick of rearranging the decor. This changes previous territories and can resolve conflicts. Make sure there are plenty of hideouts for everyone, even if that means adding more plants, rocks, and other decors.

If the aggression continues, you must eliminate either the aggressor or the one being attacked.

Problems with the water from the aquarium

An increase in ammonia or nitrite levels is an invisible threat, but this common problem can occur even in a well-maintained aquarium, especially when new fish are added.

This can affect the biological balance in the aquarium, resulting in an increase in ammonia levels, followed by an increase in nitrate levels.

New fish are already under stress because they are caught, transported, and introduced to a whole new habitat. As a result, they are more likely to succumb to a modest increase in ammonia or nitrite levels.

It is also possible to change the nitrogen cycle balance when changing water, cleaning the filter, or performing any other type of maintenance.

Things will recover quickly, but fish that are already weak may not resist the stress of changes in water conditions. A chain reaction can occur as one fish dies, adding more organic toxins to the water, so another fish that are already stressed will die from it.

The domino effect may appear as simply the result of the death of weaker or older fish who have fallen victim to what is happening in the water.

Tracking water parameters will help keep you alert about such changes and help you identify which is the real fish eater.


A disease can also be responsible for a fish’s death. Not all diseases are obvious, which is why quarantine of new fish is recommended.

Maybe that newly bought fish was already infected when you introduced it into the aquarium. Due to the stress of being moved and its illness, the fish died the night after you brought it home.

The other fish discovered the corpse the next morning and began to feed on their new breakfast buffet. Now, everyone is infected as well.

One by one they get sick. Is any of them the culprit? No, but if you find a fish with a piece of another one stuck in its mouth, you’ll probably ask yourself that question.

That’s why it’s so important to keep your new fish in quarantine and closely watch everything that happens in the aquarium. Keep a journal to know the normal behavior of all fish.

Track pH, ammonia, nitrites, and temperature so you can see if something changes in a not-so-beneficial way. Be even more careful whenever you add a new fish or make a major change in the aquarium.

There is no possibility of losing any fish when you know your aquarium well. If a fish dies, you are more likely to immediately know why or who is responsible.

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