My theories to losing lots of fighters at this age -
bacteria
ammonia build up in the tank
lack of properly developed ventrals when they start to come up for air
cold air (pneumonia)
insufficient feeding
Bad development of ventral fins (as in no ventral fins or only one)
water quality not good enough during development
nippy young fighters will often take off anothers ventrals
genetic (you can use a fish without ventrals and still produce perfect fish)
There are probably another thousand theories out there but these are the ones I have, in the past, observed.