I have (roughly) an 18" cube tank that I am putting together. I'm going to be running black diamond 20/40. How much osmocote should I run UNDER my substrate (ie will be on the very bottom) if I intend to put an inch to 4 or 5 inches on top of it? I know this varies based on intended substrate thickness, just looking for a ballpark. Or should I be considering some other solution altogether?