During a recent business trip to Europe, I spent a leisurely afternoon walking around. I stumbled upon a pet shop and went inside. There were a number of very good quality discus fish available. As I admired them, someone asked the store attendant why they were chasing each other. His response was that discus were cichlids and like all cichlids maintained a strong pecking order. That part of the response was correct. Cichlids are territorial and can be aggressive.
The next part was erring. He told the lady that male discus are always the aggressors. This is not invariably the case. If you remove the most aggressive discus fish, the next in line will take its place, irrespective of gender. Continue doing this until only two discus are left, and one will harass the other often until it perishes. This experiment would indicate, if the comment was correct, that all were male discus but one. Yet the set ratio of discus is not so heavily skewed towards males. In actuality, the gender ratio of discus fish is almost equal.
For many years it was also believed that the more colorful tropical fish from wild populations were dominant males. Dissection of colorful fish has demonstrated that this is not always the case. Breeding has also proven that the colorful individuals can be either a male or female. Particularly striking fish appear in both genders. These more colorful fish are often the more aggressive in a group.
It’s Not All Bad
Fish aggression, which can be represented by chasing, body slapping and picking, is a sign of health. Sick discus fish never display aggression. A healthy fish may intimidate a sick fish, but two sick fish will not chase, nip and threaten one another with body slapping. Sick fish are far too busy being inconspicuous. These tropical fish have inherited the survival trait that if they were sick and conspicuous, a predator would eat them; sick fish tend to be more sluggish. Hiding allows the body to recover from illness and for the individual fish to survive.
Fish aggression can be the cause of illness. A tropical fish low on the pecking order will receive attacks not only from the dominant fish but also from other fish in the aquarium if the group is small. This individual fish may be so harassed that it will perish. Having a sufficiently large group of fish can prevent this. I have never sold two discus fish to anyone. They either had to acquire a minimum of three or must tell me that they have other similarly sized fish. Housing only two discus in an aquarium is to court trouble; one will probably intimidate the other into illness. A larger group is better. Raising eight or even 12 discus fish together prevents an individual from being targeted and the focus of group aggression.
Establishing the Pecking Order
As discus fish mature, the pecking order is usually established and the aggression is less prominent. This can change if an individual is added to the tropical fish aquarium, where the hierarchy may change. If this is necessary, it is wise to rearrange the aquarium. Move the filter and heater and possibly add a ceramic flower pot or two. This will create barriers and confuse the group.
Fighting can be seen in a pair of discus fish. They may display bold colors, can lock jaws and even peck at one another. This can happen for a number of reasons: pairing an immature fish with one that is mature; placing an individual that is not fit for breeding in the aquarium with one that is; placing two fish that are of the same gender together; or they may bicker as to who cares for the eggs or fry.
Fighting among tropical fish is normal to a certain degree. It helps strengthen the pair bond. Pairs of fish that are not properly matched (i.e., which are not equally aggressive or submissive), will often bicker throughout the spawning process until one eats the eggs or fry. Such pairs will need to be re-paired. If this is not possible, then the pair of tropical fish may need to be separated with egg crate until both are clearly displaying signs of wanting to breed. Once spawning is over, they will have to be separated. This can be achieved by simply removing one fish. One can care for the eggs or fry.
Alternately, an egg crate can be inserted into the aquarium, creating a divider. The fish fry will swim from one parent to the other through the divider. I have used this method many times with success. Occasionally, it will not work and the other fish will need to be removed from the aquarium. This will need to be done when careful observation reveals that one fish is eating the fry as they venture into its territory. Such a fish will not normally darken or produce the body mucus that the fry will need as first fish food. This sign will indicate the need for observation when the fry first become free-swimming.
In discus fish of all ages, including breeding pairs, raising the pH can reduce aggression; in more acidic water the discus show more aggression than at a pH above neutral. Lowering the temperature can also reduce aggression, albeit temporarily. However, permanently housing discus fish at sub-optimal temperatures is to court trouble.
I always prefer not to alter the water and to find other means of reducing aggression, as described above. The natural behavior of discus fish as they establish pecking orders and test each other’s preparedness is part of what makes discus fish so interesting. Such behaviors belie the claim that they are morose and inactive.