When I used to breed kribs, I used a fungicide. Did this with my cherry barbs and dwarf platies too, though I was a lot more lax with that. I used Maroxy by Mardel, but at 1/3-1/2 of its normal strength, I did a partial water change right before each treatment, and I didn't treat for more than 3-5 days, afterwhich, I did a large water change and gave the tank a rest. I don't know if that'll help with cories, though. They tend to get freaky with meds. If it were me, I'd try upping the water changes first, or let them just practice. My kribs were pretty dumb at first too, took about 3-4 attempts before I was able to raise fry to adulthood. Stupid parents kept fanning and then eating the eggs. Fan, don't eat!
Good luck, I hope you get lucky next time.
llj
Good luck, I hope you get lucky next time.
llj