drainage

I am planning on making a terrarium out of a 7-8 gallon fishtank. I am just going to have all different kinds of flytraps planted in it. Do you have to have a way for the water to drain out of the bottom or would it be ok just to put the soil in the bottom and plant them. I was thinking about putting a layer of gravel in the bottom so the water would go into that.

I was also thinking of breaking the bottom pane of glass out of the tank and replacing it with a redwood board. Then drilling holes in the board to allow it to drain.
Any suggestions??

My terrarium is a 10 gallon tank with about 2 inches of perlite in the bottom. I keep the water level just to the top of the perlite. This has worked well for me.

well, actually, it's the same thing as an undrained pot... My terrarium is just a good mix of peat pearlite... like 1.5:1:.5 Pearlite, peat, LFS. So far, so good.