how much do i need to drain??

[Bob I]

Originally Posted by AJ_77

What thickness glass? I don't even have a center brace on my 50, and it's 20" tall. Glass is 3/8.

That is pretty standard Alan for a tank that has 3/8 glass, but for the more common 36X18X18 inch tanks which have 1/4 inch glass, a centre brace seems to necessary
Interestingly it is the total water volume that causes the presssure on the glass. The rock does not enter into the equation, as it displaces water.. Thus if he has a lot of rock he may be able to get away with removing only a small amount of water.