Personally, a Hexagonal 40G would be the last tank I would use to keep SW. I may consider using one to rear fry, but that would be all. I just don't like the looks of them, and unless you do something like a MH Pendant, they are a pain to light. So, IMO, that would rule out a Reef were I to do it, and all that would be available would be a FO, and a 40G seems to small to be able to keep many fish, so you would be limited to smaller fish, and in smaller numbers. You could keep a Flame Angel, Damsels, Clowns, and other smaller fish, but not that many of them either. So, if I were you, I would get another tank at all possible, because I have no idea as to how to set up the PHs, Overflow, and Spray bar and such to correctly distribute flow, since the hexagonal shape will make some odd angles. But, should you find out a good way to set up the equipment, I would add about 60 lbs LR, and 4-5 inches of LS, and set it up in a way that it is like a single Reef Head, surrounded by the vast open ocean, and then get a family of 4-5 Perculas, so that it is like their own little reef, similar to in nature. Then get an assortment of scavengers and snails and such, to be your cleanup crew, and then you'll be set.
What I mean by the LR setup, is to place it in the exact center of the tank, leaving about 5 inches of sand around the outside around the whole thing, and set up the LR like it is a ball, and build it up, but not any further out than you started. I do not know if I am correctly wording it as a Reef Head, but that is the word that was used on a Discovery Channel documentary about the life and such of clownfishes and their natural habitats.
