I use my Canon 5D mkiii with Canon's 100mm lens. The actual camera's not so critical, I could get very similar results with almost any SLR, though the high megapixels of the mkiii help if I want to crop in closer in post-production to get even closer to the subject. The lens I love, super sharp.
Photography with saltwater aquariums has its problems to begin with, add a macro lens to the mix and it really takes some practice to get good pics. Many people new to macro photography can get worse photos with a macro lens than a kit lens, until you get used to the quirks of macro photography.
#1, depth of field. Macro lenses have very shallow depth of field to them, especially if you use a wide open aperture (low f-stop number). Shooting at something like f2.8, almost nothing is in focus, your focus needs to be dead on if your principal subject is going to be in focus. This makes hand holding macro photography especially difficult, as the time between focus to shooting the pic it is very easy to sway just a tiny amount toward or away from the tank, moving your subject slightly out of focus, especially at lower f-stops. I can hand hold and get decent macro shots with a lot of practice, but to begin with when getting used to your macro lens I highly recommend keeping the camera on a tripod. That way you can eliminate the possibility of camera movement as a problem in the photography. It helps narrow down what you need to experiment and play with to get better pics. The shallow depth of field of macro lenses also makes autofocus problematic sometimes. If the autofocus is off by even a tiny amount, your subject may not be in focus. Shooting on a tripod with manual focus helps make sure you nail it for great shots. If your camera has live view, you can usually zoom up to help with fine manual focus control.
#2, lens quality. You're magnifying tiny objects, and especially if you crop after taking the pic, poor glass will not be able to give that super sharp quality of a great lens. Always depends on this, brand helps but 'bad copies' of good lenses exist too, problems with the glass or focus issues. Also keep in mind, while many macro lenses are f2.8 that sounds awesome to get lots of light (but has the depth of field tradeoff listed above), shooting at aperture extremes for any lens will have quality dropoff. Low f-stops and high f-stops will have a little less quality on good lenses, and a LOT less quality on cheap lenses. Finding your lens' "sweet spot" helps a lot, to know not to go under or over that aperture range. The midrange aperture values of lenses will give the sharpest results. There are sites like dpreview that do detailed reviews of some lenses, with graphs showing the quality falloff at different apertures and toward the edges of the lens, to help you figure what settings to use.
#3, distortion. Shooting through the side of an aquarium can be difficult. Macro lenses magnify any issues more than regular lenses, shooting up that close. You need to keep the lens at perfectly 90 degrees to the side of the aquarium, any side to side or up/down tilt will begin adding distortion that is much more noticeable with macro lenses than with regular lenses. This is especially critical when adding extension tubes to a macro lens. Dirty/hazy glass is terrible. I made a viewer specifically for my macro lens, that keeps a thin piece of acrylic at a perfect 90 degree angle to the lens to add almost zero distortion for top down pics. The sharpness is crazy! For example, a zoa pic I took, this guy is less than 1/2" across. With no distortion the detail was awesome, you can hardly even see the speckles by eye:
Remember with extension tubes, they hold no optics. They add additional magnification to the lens being used, with some significant downfalls. They make a macro lens's shallow dept of field INCREDIBLY more shallow. With all 3 of my extension tubes on my macro lens, at f2.8, I have a few millimeters (if that) of depth of field, the rest is blurry. They cut down the amount of light getting to the sensor, coupled with the increased shallowness of depth of field, you really have to crank the f-stop up to get a decent amount of the image in focus. But to do this you have to either use a higher ISO with more noise, or slower shutter speed with more potential for motion blurring. Hand holding extension tubes with a macro lens unless shooting in crazy bright conditions is really out of the question for tack sharp results. As extension tubes have no optics, the added magnification they bring will make any imperfections in the lens itself that much worse, as well as distortion from shooting through glass/acrylic. They move the focus point of the lens closer to the sensor while making infinity focus impossible, so subjects in the back of the aquarium you may not be able to focus on at all. With all 3 extension tubes, the furthest point I can focus on is about 7-8" away from the end of the lens.
Other items: ISO, shutter speed. Higher ISO while letting in more light and letting you use a smaller aperture (larger f-stop number) for more depth of field or faster shutter for less motion blur, adds noise to the image. Especially when you're trying to focus on super fine details of macro shots, noise can be quite destructive at higher ISO's. This varies from camera to camera, you'll have to play with your camera to see what level of noise you're willing to accept on your images to know how high you can go with the ISO. Shutter speed is also critical, especially when focused so close with macro lenses. The slightest amount of movement can add motion blurring to your images, even a tiny amount will ruin the sharp macro detail. Remember it is far better to use a faster shutter speed and have an underexposed image that you can lighten on computer, than a slower shutter speed that you get motion blurring with. Nothing truly fixes motion blurring, there are a few programs that can help a little but fine detail is gone forever. Also remember, when shooting with a close up macro lens (especially with added extension tubes), camera shake can be a real problem. Especially at slower shutter speeds, even with a camera on a tripod, camera shake is something to be aware of. If I shoot using slower shutter speeds, I always use mirror lockup, as even the shutter opening to take the pic can cause a tiny amount of motion blur. Mirror lockup will open the shutter when you hit the shutter once, and take the pic and close when you hit the shutter a second time (also great to use for slow shutter speed pics like waterfalls and fireworks). A shutter release cable helps here too, as manually depressing the shutter can also add some blur. This isn't as critical with just a macro lens, but add extension tubes into the mix and unless you have a super strong tripod it is a real issue.
LOL, didn't intend that to be that long
