Why are Discus Fish so Aggressive??

Discus Fish are considered a good community fish.  They very rarely will chase or bully other species of fish in the tank.  The one exception I have found is they sometimes do challenge Angelfish, but by far and away, most of the time they ignore them.

In an established tank with no new Discus added in several months, they will usually be generally calm around each other and the only interaction will be a larger one chasing away a another one that comes to close and is smaller, but close to the same size.

There are two exceptions. 

First, when two Discus Fish are getting ready to spawn or have recently already spawned, they will aggressively chase away any fish that get into their spawning space.  The male is the most aggressive as the female will be  cleaning the spawning surface.  Once the eggs have been laid, the roles reverse and the female becomes the most aggressive.  This is just a general rule and the aggressiveness can vary from couple to couple.  While they will chase any other fish away, they will most aggressively chase away other Discus, especially males.

Second, when the pecking order is disrupted, they will bully until the pecking order is again established.  Firmly establishing a pecking order can take several months, sometimes even up to 6 months.  Most of the fighting is between fish of the same or close size.  Large Discus generally will mostly ignore other Discus Fish that significantly smaller than them.  That is one reason why we often recommend adding smaller fish and not ones that are larger or of the same size.

The good news is, while the fighting may seem intense, Discus Fish will almost never kill each other.  The biggest concern is that the smaller ones do not get enough food.  We recommend that you feed at both ends of the tank at the same time. We also recommend that one of the feedings is at night with the lights off.