I had been trying to breed corydoras for a few months without any luck. The fish would spawn but none of the eggs would make it. I happened on an article about the use of leaves in the aquarium so I gave it a shot. It worked great! After I added the leaves the eggs stayed healthy and viable. Turns out that the leaves had an antibacterial aspect that kept the eggs healthy.
I used oak and walnut leaves. I just picked the dry ones from the ground. I added them to a five gallon bucket with water for a few days to a week. Then just threw them in the tank.