Pleco Has Become Very Aggressive – Reasons and Solutions

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Plecos are highly valued tank cleaners due to their scavenging potential, lay-low attitude, and overall hardiness. They are also notorious for being peaceful, tolerant of other fish, and rather shy in general.

They prefer to flee and hide than stand their ground in the face of more aggressive or bullying species.

Today, we will discuss pleco behavior and how it can speak volumes about the fish’s comfort, its relationship with its tankmates, and even its environmental conditions.

Are Plecos Aggressive Fish?

Plecos are also not particularly territorial and don’t display violence towards their tankmates.

So, it’s unnatural for plecos to showcase violent tendencies, especially regularly.

That being said, they can turn violent at times under specific circumstances. Let’s see what those are.

Reasons Plecos Become Aggressive

Plecos can turn aggressive towards each other or other tankmates in specific situations, some of which need addressing fast.

These include:

Food Scarcity

Plecos are very good scavengers, capable of consuming virtually anything so long as it has some nutritional value.

We include here detritus, algae and plant matter, insect larvae, fish eggs, microcrustaceans, various worms, etc.

These herbivorous fish prefer plant-based foods, but they won’t reject live foods if available. The problem is that many inexperienced aquarists believe that plecos can do just fine thanks to their scavenging behavior.

These fish will eat anything that falls on the substrate, including food leftovers that other fish miss.

This being said, plecos still require extra feeding to remain healthy and full. Otherwise, they might experience nutritional deficiencies and even become competitive and aggressive towards one another.

The less food there is available, the more aggressive and territorial the plecos will become.

You should always have a proper nutritional plan in place for your plecos. Two plant-based meals per day should supplement the catfish’s diet quite nicely and keep its temperament cool and friendly.


Territorial fights aren’t typical in a pleco community unless there isn’t sufficient space for all of them. You need at least 20 gallons of space for one 5-inch pleco and increase the tank’s size by 10 gallons for any extra catfish.

This will provide the catfish with sufficient space for them to go about their normal lives without feeling overcrowded.

Overcrowding will cause a variety of problems over time. Your plecos might become stressed, struggle with higher amounts of bioload, and experience higher ammonia and nitrite levels.

If you want more fish, consider upgrading the tank first.


This is a common cause of aggression in plecos and all fish in general. Fish stress isn’t a disorder in and of itself but a symptom.

This means you should assess the fish’s situation carefully to identify the triggers which will inform you of the necessary treatment approach.

Some of the common stress triggers include overcrowding, health problems, improper diet, aggressive tankmates, improper tank layout, low oxygen levels, etc.

Unstable water parameters can also contribute to the problem, as plecos require more or less stable water conditions.

The treatment’s profile depends heavily on the diagnosis process. You must first identify what’s stressing your fish and approach the situation from that angle.


Plecos, males especially, are notoriously aggressive during the breeding season. Especially towards one another.

You should always make sure that your plecos have sufficient space and plenty of hiding spots to mitigate their aggressive tendencies during breeding.

Most importantly, never have more than one male per tank. Pleco males are generally aggressive towards one another and will showcase even higher aggression when mating.

Many of them may even become violent towards other tankmates as well.

Finally, there are some rare cases where one pleco’s aggression won’t be tamed. In that case, consider removing the aggressor from the environment for the sake of the other tank occupants.

Will Plecos Kill Each Other?

Yes, it can happen. Plecos are generally peaceful towards one another, especially when young.

As juveniles, plecos will get along just fine but expect the situation to change pretty soon.

Adult plecos develop more visible territorial tendencies and tend to compete over food, females, and other valuables.

In extreme cases, plecos can actually kill each other. This generally happens over territorial fights, if the plecos are underfed, or during the breeding season.

To prevent this issue:

  • Avoid keeping 2 pleco males in the same tank
  • Provide your plecos with more space and a varied layout with plenty of hiding areas
  • Increase the tank size
  • Monitor your plecos’ interactions to detect aggression in time
  • Remove the aggressor(s) if nothing else works

This being said, plecos are peaceful towards each other under normal circumstances. Just keep in mind that they are social animals that will interact with each other frequently.

Many of these interactions may seem violent or harassing, but this isn’t always the case. So, always assess your plecos’ behavior thoroughly before looking for solutions.

Can Plecos Kill Other Fish?

Yes, plecos can kill other fish, but this isn’t the right question. The right question is: will they kill other fish? The answer is most likely no.

Plecos will almost never attack and kill other fish unless extreme factors are at play. These include a poor diet, causing the pleco to starve, bad tank conditions causing high stress, or drastic differences in size.

Plecos may be herbivorous, but they can attack smaller fish if given a chance. Especially if they are hungry and can’t find anything else to eat.

This being said, be wary of people claiming that they’ve seen plecos eating other fish frequently. This is extremely rare and is mostly indicative of poor water conditions and high pleco stress.

Most likely, the fish are already dead, and the pleco eats them as part of its scavenging diet.

Plecos eat anything dead and are more likely to consume dead fish if they come across them.

If you notice your pleco eating fish:

  • Measure water parameters immediately – You’re interested in assessing the ammonia, nitrite, and oxygen levels. Temperature and pH are also important.
  • Assess your pleco’s diet – Your pleco might not have sufficient scavenging material, in which case you need to complement its diet more thoroughly. Increase the meal size or frequency and see whether that affects the pleco’s behavior.
  • Make sure plecos aren’t breeding – Plecos are known to exhibit a bit more aggression when breeding. Rule that out before moving forward.

If your pleco eats other fish, that’s because it’s either stressed, the fish are already dead, or the catfish isn’t fed properly.

Other than that, plecos don’t eat live fish on purpose, given that they are herbivorous scavengers.


Plecos are notoriously peaceful animals, but this isn’t always the case.

Fortunately, you can now identify the triggers behind their aggressive behavior and apply the right corrections in time.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
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