I seemed to find each spawn is unique, and each fish grows at it's own rate of development even when kept in the exact same conditions as its siblings. I don't think Mother Nature intends for all possible 100-300 fry to live, that's why they have so many.
In the wild, some are eaten, some are killed by the stronger, some are just retarded, and some have accidents.
My first spawn of over 100 all died over a period of a month-month and a half. I had 2 "survivors" which I put in a low current, pristine cycled planted 10 gallon tank and they died promptly.
I was immaculate with the wc. I kept the temp at a steady 78 degrees. I siphoned, turkey basted, and removed snails and algae. They all ate well, even hatched BS for them-fed 2-3 times a day, then cleaned up after them.
I'm not countering any one's techniques, but I am beginning to think there are other factors pre-determining the fate of the fry beyond our control, in our control settings.
I bred the same fish 5-6 months later, and all are doing great at 15 weeks old.
(although, I had to separate and I'm still separating the aggressive, but that is a giving with bettas)
I was breeding for HM's, not Plakats, so I don't know if it makes a difference or not.
Just reporting my experiences