Sorry but I must disagree with you Colin. I have first hand experience with micro worms causing missing pelvic fins. It was my fault really, I wasn't cleaning up after each feeding so the left over micro worms rotting at the bottom caused their pelvic fin buds to "burn" off. Not all the fry were effected but most of them were. Some have only one pelvic fin, some have none, and a few have stubby little fins. It is
not genetic, both parents have perfectly functional pelvic fins and another batch from the same father but different mother have perfect fins.
I've heard it happening before and will link something here:
https://bettaboy.wordpress.com/2017/08/01/why-my-betta-fries-got-no-ventral-fin/
This male had 2 stunted pelvic fins
View attachment 89683
These boys have none
This female has one perfectly normal fin
Brother from another mother has 2 perfect fins