The best macro lenses for reef shooting are definitely the 100 mm lenses because you can capture corals that aren't close to the glass but if you want go cheap i recommend nikons 40mm macro lens. The only problem is its hard to shoot corals that are far back in the tank so you have to move them up close the glass to capture all the detail. I usually shoot with a high aperture around f16 and a fairly low shutter speed around 1/100 with an ISO of 200(to capture fish or other fast moving livestock). For corals ill shoot around a really high aperture to get as much of the coral in focus as possible, shutter speed will be low enough for you to need a tripod or something to stabilize the camera, and ISO will be around 400 any higher and you start getting a lot of grain. Reef shooting isn't clear cut like other types of photography, lighting is the biggest issue and cause you to get colors in corals that just don't look right so it requires a lot of playing around with settings and a lot of patience.