What would you get custom on a tank?

[Salty Cracker]

I like the idea of the slope, but does substrate flatten out over time?

 

[Duke]

probably i was meaning with your rockwork more than the sandbed, you want your taller structures at the back of the tank and the lower structures at the front generally speaking, i think only 18" inch of depth wont give you much height to play with.

 

[spyd]

With 4" of sand, I would go with a depth of 24". Any decent light will hit the bottom with no issues. I promise you that you will be extremely happy with an external overflow. Just the additional space and the nice clean look is enough to go that route.