I've taken a lot of macro and closeup photos, mostly in a 120, with the Canon 100mm. Yeah, the stuff at the back of the tank wasn't quite at 1:1, but it was pretty close and I'm happy with the results.
I've been under the impression people tend to not use a flash when taking macro shots, but only use the lighting the tank provides.
I gotta tell you, I spent the first six months (or more) shooting just with tank lighting and I'm not going back. You'll never get the sharpness or level of detail that you really want. You'll wind up taking 30-40 shots just to get one that comes out ... well ... okay and that's if you're using a tripod. But you'll never get the _really_ nice shots (like the ones that Greg and a few of the other people around here take) without working supplemental lighting into the scheme.
That ricordia shot up there was handheld, as was the snail and spider. I'm not saying that my goal is to handhold all my shots. The coral and snail photos were taken in a LFS and I didn't want to impead their flow of customers too much. But, it's just so much easier if you get some flash coverage so you can use a smaller aperture and quicker shutter speed. I will consider shooting some of the Acroporas without flash because I've found that the flash brings out the browns and suppresses the beautiful colors in those corals. (BTW, can anyone tell me how to avoid that effect when using a flash with Acroporas?) But, I'll also consider shooting some different corals instead.
What I was talking about with the distance number above is the closest distance that the lens can focus on a subject. For the Sigma you're talking about and the Canon that I use, if you twist the lens so that it's focusing as close as it can go, whatever you are focused on without changing the lens will be at 1:1.* You _can_ still twist the lens out and focus on things that are farther away, but they will be smaller than life size when you take the picture. But, with the Canon I think the closest focus is 6"-8" or so. With the Sigma I think I saw that it's 15". At any rate, whatever that number is, you have to be at least that far away from your subject to focus on them.
*By the way, I've oversimplified some of my explanations for you a bit. There are also other factors at play such as the effect of my camera (I don't know what camera you have) having a sensor that's actually smaller than the size of 35mm film. So, when I'm taking a shot that should be 1:1, I'm actually already taking a photo that's a little beyond lifesize. And, my 100mm macro acts like a little longer lens for my camera. But, all that is a bit much for this lens conversation. I just wanted you to know that the things that I'm saying have an implied asterisk with a great big footnote that should be around here somewhere, but they are technicalities that we can avoid unless you're really interested.
