Bristle nost catfish question

[Silver Dollar]

I was really pleased when my BN bred, the first time there were 24 fry and the second time, very soon after the first hatching, there were 15. But, when about three weeks old and all doing, apparently, well, the whole batch died within 24 hours and I still do not know why. PH seemed OK, the ammonia was showing nil. Any ideas?
Waterheater

 

[Neal]

Alot of my mollie fry died at once, and I figured it was oxygen. It could be to do with that, alot of fish die from lack of oxygen. You could maybe buy an airstone or an airpump or something similar. Sorry to hear about your losses though.
Regards,
Neal