How many times will I have to change my water before it will it will take care of itself for a longer period?

I have been finding corn kernels in the substrate when I sweep the gravel, and it has a nasty, rotten smell to it. I assume it was dyed along with the substrate because it was blue, the same color as the gravel. Why would the manufacturer put corn kernels in the substrate? It is putting nasties into my water, I find a kernel about every time I sweep the gravel.