here is what I had to work with when I was doing my 50G inwall:
So all finished it looked like this behind (and under) the tank:
my fuge is what was tucked under the stairs
I recently (over xmas) moved the fuge out from under the stairs..
I like it better now, as I can sit on the floor and stair into the fuge and see cool stuff and critters! plus its easier to work in, and closer to the sump...