With the experience with my pair of jags, the frys never live pass a few weeks. My male is about 10 inches and female about 8.5-9. They've been spawning since 4 inches. For some reason the pair keeps attacking their reflection off the tank and im assuming they think its a threat so after a few weeks the frys dissapper. Then they repeat with spawning again a week after of when i do a water change. If i want to keep the frys i usually have to net them out and raise them seperately if i want to keep them.
With frys dying randomly, some fry will die off slowly if not feed with live hatched shrimp. I only used hikari first bites and some fry won't take it so they starve til the die. But with the right water parameters and newly hatch brine shrimp most fry will survive.