I know a decent amount about photography from my B&W SLR Film days, so I am good with photographing as a skill, but have very little experience with macro lenses, and its hard to take knowledge of normal photography, and apply it to photographing coral without trying things out on coral.
I can ask my Canon shooter friend what he recommends - I think he'll say the 150 or 180mm - guess it all depends on how far away from your subject you want to be
I've been looking at a 60mm micro myself or a 105 because my tanks not that big/deep - I have a 300mm F4 lens that I've got a Canon 500D close-up filter for that I use to shoot macro bugs and plants outdoors - but I still need to be 20 inches away from my subject to get it in focus (kind of hard to work with indoors shooting my tank)
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