Selecting a good sanke, when the fish are 4-8 inches in length, is very similar to selecting a good kohaku. The head of the sanke, as in all varieties of koi, is one of the most important things to look at. The head should be the exact same head as you would buy in a kohaku. There must be red on the head and the red should go down to the nostrils. The nose is better if it is white. One eye may be covered in red as long as the red is not over-powering. Red is too over-powering when it covers both eyes. THERE MUST NOT BE ANY BLACK ON THE HEAD.
Next look for an interesting pattern of red all the way down the body. Do not pick a koi with only red on the front half or on the back half of the body. If you have followed the above steps, you will notice you now have a good kohaku.
Now we must add the black. It is best if the first black spot is on the shoulder (not on the head). If there is no black until further back on the fish, the head can look too elongated. The rest of the black further back on the fish should be in a nice pattern that is balanced and preferably in the white areas rather than on the red. Some black on the red pattern is alright but the more black in the white the better. The black spots are better if not much larger than a quarter and should not distract from the red pattern.
Modern sankes are basically kohakus with a few small black spots placed in the white field. Red should take up about 70% of the fish, the black about 10%, and the rest white.
At an early stage, look for a fish with a lot of interesting red and very little black. Black tends to come out later and if you start with too much black, the black may be too powerful later. The red may be bright red or orange-red because the red can improve with age. The white should be snow white or slightly pink white. Black develops later, so pick one with only a little black that is balanced now. Avoid any sanke that has a lot of small, black, peppery like spots. Pepper like spots can be a sign of poor quality.
The fins on the sanke should be either all white or white with black stripes. There should be no red in any of the fins. Be careful buying an early sanke with many black stripes in the pectoral fins. These sometimes develop too much black at a later stage.
Again, as in all koi, look for broad shoulders, large skull, and large pectoral fins for good growth potential. Make sure there are no defects or disease on the fish. Happy koi keeping.