Intraspecific agonism is a problem in fish culture. This has corresponding fish welfare implications. As the design of the holding container may moderate aggressive behaviour, the aim of this study was to reduce agonistic behaviour by altering tank dimensions without changing the available volume. The study used swordtails, a popular aquarium species in which dominance hierarchies between males are common. The frequency of attacks initiated by dominant males against subordinate males was significantly reduced in tanks with smaller surface area (t(12) = −3.849, P = 0.002) while the frequency of mating displays, assumed to be a measure of normal behaviour, remained unchanged (t(12) = 1.126, P = 0.282). The time that subordinate males spent feeding was also unaffected (t(12) = −0.081, P = 0.937) while dominant males reduced the time spent feeding in tanks with smaller surface area (t(12) = −3.468, P = 0.005). However, this is unlikely to be detrimental to the health of dominant males but may reflect a decreased food requirement through reduced energy expenditure. Alteration of tank dimensions in this way is therefore a simple means for small-scale facilities and pet stores to reduce agonistic behaviour in swordtails.