This could be due to several things:

1.) Lack of calcium/nutrients for the parent fish. If this is the case then I suggest feeding them either something with ground krill in or daphnia.

2.) Just a genetic defect. If this is the case then the next brood from the same fish should be normal.

3.) As you said, too many fish in one brood. If this is the case then the next brood should be smaller.

I hope this has helped.