By the way when I changed out the substrate in my 10g tank...I did 100% at once (not too hard in a 10g) and had no ammonia or nitrite spike at all. I think it has to do with how fully loaded the tank is fishwise and how good your filtration is. With better filtration, more of the bacteria will be in the filter than the substrate so the chances of a minicycle drop somewhat. In my case, getting rid of my substrate which was caked with living or dead bga and other algae of various sorts, and hiding a lot of yucky stuff which got removed when the substrate came out, actually reduced the bioload of the tank and possibly offset any minicycle I might have had otherwise.