I think Garuf is right. lol
It's great to see Tom dispatch myths and he does it well even for those of us who don't understand the science fully we can still see it.
It is even better though when someone wants to argue the point with him and Tom makes sure that he shows who is on the right course.
I also like how Tom makes clear if it is something he assumes or whether it is something he knows too. I've recently seen some great posts where Tom clearly accepts that it isn't something he has researched 'yet'

and gives the person suggesting the 'theory' a maybe your right kind of answer. lol
On the original substrate question I have had my Tropica in since September 2007. Been through 2 rescapes and in a couple of months I shall scoop a load of the sand off the top. Give the Tropica a good stir and the put gravel back on top of it
AC