defects in fins

I have just had to cull an entire batch of Diamond Koi fry, which is never nice. They had an issue where the Anal fin was heading in line with their tail. I have never seen this before and wondered if this is genetic / parastic/water related. does anyone have any experience with this?

I've had this problem over the years, the suspected causes are bacterial/fungal and type of anti fungal agent used. The 'damage' is done early in the fry's development, the use of methylene blue as an anti fungal coincided with some instances of this problem. Another possibility is bacteria/fungus in the area where the newly hatched fry spend most of their time (tank base), this fin is closest to the substrate (and the lower rays of the pectorals, that's why they bend downwards on some individuals) and its development is impaired. I've not had this problem since using small bristlenose plecs to keep the fry tanks clean and using acriflavine as an anti fungal. HTH.

Hmmmn
now there seems to be a common theme with the MB !!! Will try Arctiflavin a try in future

I've never had a whole batch with this problem, but have seen it with individual fish.
I never use meth. blue, but suspect it may be associated with bacterial or water quality issues.
One of the reasons I use quartz gravel in my breeding tanks is that I feel it keeps the young away from the bottom of the tank. Similarly parent raising helps keep the young off the bottom.