I have 16" RB over my 29g, I'd say it just barely covers it, on a 40b you will probably have somewhat noticeable dark spots on the sides. Photon 24 is cheaper then 2 of the values, its what I think I'll be putting over my 40b when upgrade the lighting. Plus the controller will be nice to have I kind of regret going value over the photon for my 29.