if you have pointy bits of gravel then forget about corys..... it will damage their barbels and can lead to infection, its just not fair on the corys!
people are advising you to add sand, you dont need more powerfull filters just have the inlet a couple of inches above the sand and you will be fine.
it sounds like you planned the tank for a month without your stocking in mind.....!
I did have some stocking in mind. I built this tank around zebra danios. Ill admit the corys are an after thought.
However, I came here for advice on two species, not to lectured into changing to sand, despite telling everyone twice I'm not going to. And definitely not to be blasted for "not having stock in mind." This is why I was hesitant to even ask. Forget I even asked. I will go with what I want since no one really seems to want to help, and seems to only just attack me for my choice. So, don't bother answering anymore, as I shall be leaving this forum for good.