Start with water, then behaviour
Hold the aquarium inside 22–28°C and pH 6–8 without chasing a precise number. Every candidate needs a meaningful overlap, but overlap alone is not compatibility.
Reserve the adult footprint
Plan at least 200 litres and 120 cm of swim length for the focal species before adding the spatial needs of companions. Territory and floor area can become limiting while a conventional bioload estimate still looks comfortable.
Schooling and social structure
Meet the focal group guidance of 3 before creating a collection of undersized groups. For territorial species, use planting and hardscape to break sight lines and keep a separation plan ready.