Buying your first koi is more nuanced than choosing a fish by colour alone. Japanese breeders evaluate koi on body shape, pattern, skin quality, and genetic lineage — and the fish that will develop best is rarely the one that looks most striking in a tank at 20 cm.
Body Conformation First
Before pattern or colour, look at the body. A koi should be torpedo-shaped — broadest at the shoulder, tapering evenly toward the tail. Avoid fish with a pinched area behind the head (suggests stunted growth), a humped back, or a thin, laterally compressed body. These defects do not correct with age. A plain-looking fish with excellent body shape will always outgrow a patterned fish with structural flaws.
Skin Quality
Hold a torch to the side of the fish in a white bowl or bag. Good skin has a deep lustre — light seems to come from inside the scale rather than just reflecting off the surface. This quality is called 'tsuya' by Japanese breeders. White areas should be bright white, not grey or yellowish. In Kohaku, the white ground is the canvas everything else sits on — poor white kills a good pattern.
Pattern Evaluation
For Gosanke (Kohaku, Sanke, Showa), look for pattern that is balanced from head to tail. A stepping-stone pattern — distinct red or black blocks with white breaks between them — is generally more desirable than a continuous blob. The head pattern matters most because it faces you most often. A small, clean hi marking on the head is better than a muddy large one. Avoid fish where pattern runs over the eyes or heavily covers the nose.
Colour Depth and Edge Quality
Red (hi) should be deep and even — a uniform brick-red rather than orange or washed out. The edges of each hi plate should be crisp and distinct. Fuzzy, feathered edges suggest the pattern will continue to bleed as the fish grows. Sumi (black in Sanke and Showa) that is sitting in the skin rather than 'floating' on top will be more stable long-term — though sumi develops slowly and is hard to judge in young fish.
Buying Tosai vs Nisai
Tosai (one-year-old fish) are cheaper but require experience to select well — their patterns change significantly in year two. Nisai (two-year-old fish) have settled more and are easier to evaluate. For a first purchase, nisai at 35–50 cm gives you a clearer picture of what the fish will become. Buying tosai is a higher-risk, higher-reward strategy best suited to keepers with a dealer relationship who can see both parents.
Female vs Male
Females grow larger and rounder in body, which is generally preferred for show quality in Gosanke. Males stay slimmer and tend to be more active. If growth and eventual show potential matter to you, females are the better long-term investment. Males are excellent as garden pond fish where active, curious behaviour is an asset.