Discus fish belong to the Cichlid genus and therefore should be kept in groups of at least 10 fish! This is true, except when keeping a pair of 5" or larger discus for breeding purposes.
Typical behavior for this type of Cichlid is that, in smaller groups, often one fish will become dominant. This fish will then attack all other discus fish in the group, and often the group dynamic will result in the group preventing the weakest fish from feeding.
The weakest fish will then become dark in color, remain immobile in a corner, lose weight, will be excluded by the group and may die. If this happens, the group then turns on the next weakest fish and prevents it from feeding, and so on.
To prevent such behavior from continuing, we recommend that you increase the stocking density until the aggression in the group decreases. Experience has shown that discus fish behave peacefully in group sizes of 10 fish or more.
This behavior is well-known in African Cichlids. Discus fish are South American Cichlids with similar behavioral characteristics.
Stocking Suggestions and Aquarium Sizes
When buying discus fish, you should choose one of the following four options:
1. Breeding Discus (Discus Pair)
To breed discus fish, we recommend using an aquarium with a capacity of 40 US gallons.
Juvenile fish should be separated from the parent fishes when they are 12 – 14 days old. At this age they start seeking out food on the floor of the aquarium and could ingest pathogens or germs.
2. Discus Pair with Companion Fish in an Aquarium with Plant Life
We recommend an aquarium size of 65 gallons when keeping a pair of discus fish.
The two fish should also be at least 5 inches in size, because two smaller fish, kept as a pair, will not grow well. This is because “competitive feeding” does not take place.
3. Rearing Discus Fish, Keeping a Discus Group
To rear a discus group (of at least 10 fish) an aquarium with a capacity of 50 US gallon is sufficient.
However, for 10 fully-grown fish we recommend using an aquarium with a capacity of at least 80 gallons.
4. Discus Group with Companion Fish in Aquarium with Plant Life
For 10 fully-grown discus fish with additional companion fish and plant life we recommend using an aquarium with a capacity of at least 105 US gallons.