Do You Have Cannibalistic Fish?

Are Your Fish Eating Other Fish? Find Out Why

Bristlenose fish

Giles Alexander / Flickr / CC By 2.0

A fish owner notices fish disappearing, then sees a normally peaceful fish feasting on the carcass of those that have gone missing. Can your fish really be killing and eating your other fish?

In reality, seeing one fish eat another actually means very little. When a fish dies, other fish in the tank will quickly take advantage of the situation. Even a fish that is still alive, but is very weak or sick, will be picked on by his tank mates. Here's why that happens.

Why Do Fish Eat Other Fish?

Fish are opportunistic, just like other living creatures. If food presents itself in any form, they will eat it. Sure, that food may have been their best swimming buddy the day before, but today he's lunch. It's all part of that great circle of life.

So how does a fish owner know who, or what is the real killer? Sometimes it may be an obvious bully, but it's also just as likely that it's not another fish at all. A series of deaths could be the result of a change in water conditions, an outbreak of an undetectable disease, or simply stress. The key is to carefully observe what is going on in the tank. This is particularly true if you've added new fish to the tank or anytime something changes in the aquarium.

New Fish Take Time to Adjust

New fish in a tank are under close scrutiny by the other fish. A pecking order exists even in a tank of peaceful fish. Territories have already been established and all eyes are upon the newcomer to see which space he will choose. If the other fish become threatened in any way, quarrels might break out, even with normally peaceful fish.

Observe all the fish closely any time you add a new one to the tank. If you see any signs of aggression, use the old standby trick of rearranging the decor. That alters the previous territories and can settle things down. You may have to remove either the aggressor or the object of the attacks if the aggression continues.

Make sure there are plenty of hiding places for everyone, even if it means adding more plants, rocks, and other decor.

Water Problems Can Play a Role 

An ammonia or nitrite spike is an invisible but not uncommon problem that can occur in even a well-established tank, particularly when new fish have been added. This can tax the biological balance in the tank, resulting in a brief spike of ammonia followed by elevated nitrate. The new fish are under stress already due to being caught, bagged, transported, and introduced into an entirely new habitat, so they're more susceptible than usual to a change in water.

It's also possible to briefly alter the balance in the nitrogen cycle when water changes, filter cleaning, or other types of maintenance are performed. Things will usually settle down quickly, but fish that are already weak can succumb to the stress.

A chain reaction may occur; one fish dies, adding more organic toxins in the water, and another fish that is already stressed dies too. The domino effect can appear to be a killer fish when it's simply the result of weaker or older fish falling victim to what is happening in the water. Tracking water parameters will help to alert you to such changes.

Disease Can Weaken Fish

A disease can also be the culprit behind a series of fish deaths. Not all diseases are obvious, which is why quarantining new fish is recommended. Perhaps that new fish you bought was already infected when you put it in with your other fish. Between the stress of being moved and the disease, the fish went belly up the night after you brought it home.

The other fish discovered the body the next morning and quickly went to work feasting on their newfound breakfast buffet. Now they are all infected as well. Some of them may survive, but weaker ones may perish and be eaten by their tank mates.

How to Prevent Your Fish From Eating Each Other

It's important to quarantine your fish and to stay on top of everything going on in the tank. Keep a journal so you know the normal behaviors of all your fish. Track the pH, ammonia, nitrite, and temperature so you can see if something is changing in a not-so-good way. Step up your observations whenever you add new fish or make a major change in the tank.

You aren't as likely to lose any fish when you know your tank. In the event that you do lose some, you're far more likely to know who or what the real killer is.

Article Sources
The Spruce Pets uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  2. Petty, Barbara D, and William A Fraser. Viruses of pet fishThe veterinary clinics of North America. Exotic animal practice vol. 8,1 (2005): 67-84. doi:10.1016/j.cvex.2004.09.003

  3. Management Of FishMerck Veterinary Manual.