Hi smithrc
I take the substrate out because it is much easier to keep the fry tank clean if it has a bare bottom. Corys need a constant supply of food to grow well, and therefore a lot of water changes which are hard to do with sand moving around and possibly burying them.
Since cory fry live at the bottom, it needs to be kept very clean. The uneaten food and dead microworms sometimes tend to stick and build up a film which can feed bacteria that can quickly kill them. You can't see it on sand, but it is quite visable on the glass. It can be easily removed by just rubbing the glass with your fingers and flushing the tank with clean water.
fair enought
thanks
Russell
Hi smithrc
I don't think it bothers the corys much to have a bare bottom, but they will spawn on the glass there, just like they do on the sides of the tank. Then, since they are active and sit on the bottom, you are apt to find that the soft, newly spawned eggs, on the bottom, are just smashed. They don't realize what they are doing or know to avoid sitting on them.
I've never known corys to spawn on the sand or gravel. They much prefer a smooth surface like the glass or sometimes plants.
What kind of corys are you thinking about breeding?
I have some C. aeneus (who never seem to eat their own eggs) that have been spawning in one of my cory communities and I saw the green lazers eating them, but I understand corys of any species might be prone to doing it.
Pandas are a good choice. I found that getting a healthy and mature community of them together was the hardest part. They spawn a small number of eggs at a time, but if they are well fed, will do it on a regular basis. I rarely see healthy looking ones at the lfs, and had a hard time getting my group together. The ones that are for sale, and they do have them often, are either so tiny that the odds of them reaching adulthood are not all that good, or adults that have been shipped in from somewhere and look weak and shaky. 
