The tops come off in two ways...If you look, you'll see a little break in the Handle on top...that is an "inner circle" that you can spin off, and then the large black circle is the "outer circle" that also comes off. They also have a bulkhead pre-installed on the bottom of the opposite side to my picture.
I don't think they are stackable...I got them from Corix and they are made in Vancouver. Really heavy duty. I was going for other ones from Andrew-Sherat but these were just way better built. Plus the shoulder of the tank allows me to put in some additional plumbing with uni-seals without having to worry about slopes (a straight spot to drill a hole)
They will sit side by side on the small stand you see at the back of my fish room in the pic below:
I wanted just enough rise that gravity would empty them. I plan to do a fully plumbed water change system to the sump which will sit on the stand to the left. There is also a dedicated drain over there in the floor that you can see (black pipe)