If its 40 gals then its about the same as my Tang set up,you could go for a small harem of "shellies" Multis would prob be the best for a tank only 3ft long,go for 3 or 4,1 male to 2 or 3 females ,and loads of shells,Julidichromis or "Julies",Ornatus or transcriptus are the smallest julies they will suit your tank best,also Calvus are good addition white or black whichever you choose you wont be disappointed