Small tanks make cichlids angry

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If you think your aquarium is too small, maybe it is time to consider anger-management therapy for its inhabitants, according to a study published in the most recent issue of the Journal of Applied Animal Welfare Science.

Ronald Oldfield of the Case Western Reserve University conducted the first scientific study to examine the effects of the home aquarium on the aggressive behaviour of fishes.

Using the Midas cichlid (Amphilophus citrinellum) as his model, the author carried out a series of experiments in which sexually immature fish were placed in an aquarium and their behaviour recorded.  

In the first set of experiments, the aquarium size was kept constant at 38 l/8 gal, and each series of eight treatments varied in the number of cichlids kept in the tank (three, five, seven or nine).

In addition, some experimental groups were tested in complex environments that included one stone in the centre of the tank, one clay tile leaned against each end wall to create two caves, and one bunch of Java moss (Vesicularia dubyana).  

These additional treatments were conducted with groups of three and five fishes. In the second set of experiments, groups of three cichlids were maintained in aquariums of differing capacities (38, 110, 151 and 380 l) and their behaviour again recorded.  

As with the first set of experiments, the behaviour of the fishes in complex environments were recorded in tanks of 38- and 110-litre capacities.  Lastly, an extremely complex environment was created for one of the 380 l aquariums and the behaviour of a group of three cichlids also recorded.

Behavioural observations consisted of recording the number of aggressive interactions among the fishes for five minutes.

Observations on the behaviour of Midas cichlids maintained in a large enclosure (a large artificial stream in the Toledo Zoo) and in the wild (Lake Apoyo, a crater lake in Nicaragua) were also made as a basis for comparison.

In the first set of experiments, the author found that group size did not significantly affect the degree of aggression displayed by the dominant individuals, although the subordinate fish spent significantly more time cowering in larger groups.

Environmental complexity also had no significant effect on the degree of aggression displayed by the dominant individuals.

In the second set of experiments, Oldfield found that available space did not significantly affect the degree of aggression displayed by dominant individuals, although subordinate fish spent less time cowering when given more space but not when given a more complex environment of the same size.  The proportion of time spent in aggression was significantly affected by complexity, however.

Coupled with the data made from observations of the cichlids in the artificial stream and crater lake, this suggested that the cichlids displayed increased aggression as a result of the ecological conditions imposed by small aquaria that contained limited levels of habitat complexity.

This aggression is heightened by the low numbers of competitors, little available space and the potentially large benefit of preferential access to food items that might arrive in the future.

The results of the experiments indicated that only the 380 l./80 gal tank with a complex environment was sufficiently large to house small groups of juvenile Midas cichlid in relative comfort (ie. with the fish not displaying significantly greater aggression to conspecifics than in the wild), but given the large adult size that this fish grows to, even this is not sufficient.

The upshot of this is that many aquaria of sizes typically used in the fishkeeping hobby are not likely to provide optimal welfare for cichlids housed with aggressive conspecifics.

For more information, see the paper: Oldfield, RG (2011) Aggression and welfare in a common aquarium fish, the Midas cichlid. Journal of Applied Animal Welfare Science 14, pp. 340–360.

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