One suggestion I'd make is to try and go for a 24" wide rather than 18". For a peninsula tank. 18" will be very narrow and it'll be a pain to aquascape. You need to leave some room room for coral to grow and fish to swim on either side of the tank which won't leave a lot of room in the middle for an aquascape. 18" certainly is doable but I guarantee you will wish it was 24" down the road :-)
If you can swing it, mind as well do it now. I speak from personal experience of having had both an 18" and 24" wide pen tank.
