Have you used the Methylene Blue? You should add a touch of that to the eggs after they are laid, if you remove them to a separate tank. I added mine too late because I couldn't find it near me and had to go to a lfs an hour and a half away to get it. Use a small dose, not the normal dose for diseases.
Luckily, I ended up with two, beautiful baby cories, and they are now big enough that I removed the nylon off the filter intake for good last night.
I use Playsand from the hardware store for my cories, but pool filter sand is supposed to be better ( a little larger grain). My babies, however, are in a bare-bottomed tank except for a large square (about 6 to 7 inch square) plastic
perware container with Seachem Onyx sand and the plants in there. I really like how this worked, because gravel vaccing with babies and eggs is a scary thought. The plants hid the babies and took care of my ammonia issues (it is a thickly planted
perware!). Where are you located? I truly don't think Seachem Onyx sand is good for cories, as it is a sharp sand. I would stick with the round grains in playsand or pool filter sand or even Estes brand "marine sand" which is freshwater safe but expensive because it is sold in little tiny bags (still, for only one inch of coverage, it wouldn't take too many bags). When in doubt, make it thinner, not deeper).
I would suggest keeping sand very thin in your main tank (an inch, maybe). I made mine much too thick and am slowly having to remove some due to toxic anaerobic gases that can build in deep sand.
Be careful when you remove the eggs that you don't damage them. If they lay them on plant leaves, remove the whole plant leaf (that's what I did).
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