The big difference between myself and Dennis in regards to photography is that I was a high school photography teacher and he is a pro photographer. Dennis has access to and has used a wide variety of equipment and his knowledge about shooting awesome shots in a wide variety of sticky situations is unmatched. I can teach you how to compose a shot and to use a camera. My background is in fine art photography so my shots are often weird or wonky applications with a camera. He'a a better resource for you in this situation than I am but I can offer a bit of help for you.
A fixed focal length lens is a little lens you fit on the end of your regular lens. It has a fixed focal length, meaning, it has one "sweet" spot and you move your camera back and forth gently to find the proper focus. A set of lenses is extremely cheap and will give you good shots in your tank. The problem with fixed lenses is that you have to be incredibly patient and you will have to keep swapping lenses each time you want to shoot something in a different spot in your tank. If you want to shoot something that is moving, like a fish, it will be incredibly difficult. If you want to shoot corals, rocks, crabs, snails.... then you can get a good shot if you're patient. It is a bit cumbersome especially at first until you get the hand of using them. With practice, you'll get good, consistent shots. If you are ONLY shooting your tank and you're on a budget, I strongly recommend these lenses. You can also get extension tubes that fit between your camera and your lens to help your lens get closer focusing. If you can spend a few bucks then the macro lens is a really sweet way to go. They are versatile and allow you to get up close and personal with your subject matter. Personally, I hate auto-focus on my macro's and wouldn't ever spend extra money for that feature but Dennis might be able to tell you why you should get it. You want to buy a used lens from a reputable seller ONLY, there are a myriad of ways a lens can go bad and you need to do your homework, especially if you are buying sight unseen.
What lenses do you currently have?
The lenses you're looking at are regular zoom lenses that have macro capabilities. Both are discontinued. Neither are official "macro" lenses, they are everyday 'walk around" lenses that have macro capabilities. The second lens the 70-210 is listed in "fair" condition which would make me pass on it immediately. I've never used either of these and wouldn't feel comfortable giving you specific information about either of them. I don't think these are what you're looking for if you want a true macro lens. I like the Tamron macros with my canon.
Don't know if this helps?
Catherine
