It is very confusing to rookies actually.
P13 and black diamond is the same specie but with different outlook, white spots pattern and price. For P13, the white spot pattern are all the same, you can count it form eye to tail which shown as 3, 2 and 1 spot/s pattern. For black diamond, each individual has different pattern of the white spots. Traditionally, black diamond is always more expensive than P13 because it looks better and not easy to catch from the river.
P13 and black diamond is the same specie but with different outlook, white spots pattern and price. For P13, the white spot pattern are all the same, you can count it form eye to tail which shown as 3, 2 and 1 spot/s pattern. For black diamond, each individual has different pattern of the white spots. Traditionally, black diamond is always more expensive than P13 because it looks better and not easy to catch from the river.