Fish are capable of colour vision and in captivity, background colour can affect growth, survival, aggression and reproduction. In general, research has shown that fish fair better in darker environments, possibly because they allow fish to be better camouflaged and prey to be more easily detected. Unfortunately, the compatibility of fish with tank colour has been largely neglected within the aquaculture industry; common practice is to use light blue tanks but there is no scientific evidence to support this choice.  The aim of this study was to assess tank colour preferences and the effects of colour on aggression in coho salmon (Oncorhynchus kisutch), one species used in aquaculture. Salmon (n=100) were randomly assigned to 10 tanks, each bisected to allow fish to choose between two colours. Using a Latin-square design, every tank was tested with each of the following colour choices (8 choices in total): blue vs. white, light grey, dark grey, and black, as well as black vs. white, light grey, dark grey, and a mixed dark grey/black pattern. Fish showed a strong preference to avoid blue when darker backgrounds were available (p < 0.001) and a clear preference for black backgrounds over all other options (p < 0.01).  Moreover, compared to lighter backgrounds, darker colours resulted in less aggressive behaviour (p < 0.0001). These results provide the first evidence that darker tanks are preferred by and decrease aggression in salmonids. These results provide a scientific basis for the recommendation that dark enclosures be used to house farmed salmon.