in the instructions from the GARF website it says to space the bottom up 1/4".
personally i dont like that idea, like you said,
a 48x18x20 will hold around 250L of water, being 250kgs
800lbs of rock is around 360kgs
not to mention sand bead, driftwood and whatever other stuff you put into it..
ultimatly thats a 1/4" piece of glass supporting 3/4 a tonne of weight..
There is no way in heck i'd have that in my house, especially running off a 6 outlet power board which will be either directly underneath it or next to it..
Personally I don't like the garf standards they use, I don't think they thought about it very well.
Even the stand they descrive has the potential to warp twist, and crack or shatter the tank.
I'd recommend people stay away from GARF, but thats my personal opinion mate..