The fry is now half an inch (13 mm) long. Even though the tail is now more or less defined, still, it is hard to detect its details and asymmetry between the tail lobes and shoulders, and the tail split. Today my goal was to select the fry with a straight body line and with the most open tail, which ideally should be 180 degrees, but not narrower than 160 degrees. The culling was done through looking from above. Below, I put together a few diagrams to illustrate what tail shape and development I was looking for. Misshaped tail is outlined in blue and should be the reason for disqualification. The photos are not of my actual fry and are used only for illustration purposes.
The next cull should be carried in about two weeks or when the fry reaches 6/8th of an inch (20 mm), and the tail is further developed with more details visible. I anticipate even greater reduction in numbers then.