To be honest, that's a good tank for growing the young fish on in. I'd loose the substrate all together especially for fry as cory's just sit on the bottom when newly hatched and may get lost or stuck in the gravel, especially if it gets full of crap. You may want to remove the eggs to a small tank or container for hatching, then return them to the growing on tank after about a week. If you don't want to remove the eggs from the glass, you'll probably find some laid on the amazon sword leaves, I can get in excess of 50 fry of 2 or 3 leaves that have eggs on them. Just remember whatever you use, keep the water moving with an airline and keep the water temp up with a small heater. My fry hatch within 3 days at a temp of about 27C. When newly hatched do a partial water change of about 20% daily, in a small container that could be as little as one litre jug of water. In the larger tank you would probably be alright with 20% changes 2 or 3 times a week.
Good luck!