Carotenoids are routinely incorporated into ornamental fish diets with the aim of enhancing companion fish colouration which may concomitantly affect fish behaviour. Previously, colour enhancement has typically been achieved using synthetic carotenoids, however, there is now growing public demand for food additives such as carotenoids to be derived from natural sources, which can be acquired from microalgae and cyanobacteria. There has been very little research into whether natural carotenoids alter fish behaviour in a similar way to synthetic carotenoids; the present study aimed to determine whether behavioural changes typically associated with increased carotenoid consumption differed according to carotenoid source in the cherry barb (Puntius titteya). Cherry barbs were fed one of four diets (carotenoid-free, 20 ppm synthetic astaxanthin (AX) sourced from Carophyll pink®, 20 or 40 ppm of natural-AX sourced from Panaferd) over a 12 week period and then observed for colour changes, mate-choice and aggressive behaviours. The diets containing 20 ppm synthetic-AX and natural-AX enhanced male red colouration of the anal fin and anterior dorsal area, via a reduction in hue, in comparison to the carotenoid-free control diet whereas only the 20 ppm natural-AX altered the hue of female colour. In the mate choice trials, males spent more time with females fed the 20 ppm synthetic-AX and 40 ppm natural-AX compared with the carotenoid-free control and 20 ppm natural AX. Experiments conducted under red-blocking and UV blocking conditions demonstrated an effect of red colouration and ultraviolet reflectance on mate discrimination. Interestingly, males fed both the synthetic and natural AX diets reduced aggressive interactions with a mirror image, even though they displayed enhanced red colouration, which is often used by fish as a signal of increased competitive ability. In conclusion, source of dietary AX affected the behaviour of cherry barbs, to the extent that synthetic AX exerted a stronger effect on mate-choice behaviour under full spectrum lighting in comparison to a similar concentration of natural AX. This therefore demonstrates that the behaviour of companion fish can be influenced by the source of carotenoids within their food.