Macro photography is difficult to master. The 2 basic challenges are 1) depth of field, there isn't much, and 2) motion, yours, and your subjects, and because of #1 it doesn't take much. At a ratio of 1:1, which is a true macro, good depth of field is about 1/4", great depth of field is about 3/4", and good luck getting it. Depth of field, for those that aren't sure about is the apparent amount of the subject that is in sharp focus. The amount of depth of field you have to work with depends on three things.
1) Distance from subject, the further away from subject, the more depth of field you will have.
2) Focal length of lens, that's the mm number, ie: 55mm, 105mm, 18-70mm. The longer the lens, higher the mm number, the less depth of field you'll have to work with at a given distance, see #1.
3) The diameter of the aperture of the lens, or f/stop. The larger the aperture (smaller f/number ie: 2.8) the less depth of field and conversely the smaller the aperture (higher f/number ie: f/22) the more depth of field you'll have to work with.
All of the above interact proportionally.
One other fundamental depth of field rule, there is one true plane of exact focus of the film plane, it is after all only a 2D object as was film (most cameras have a scientific notation icon at the exact plane of the imager, looks like a circle with a horizontal line running thru it), roughly 1/3 of the depth of field distance in front of the point you're focused on, and 2/3 of the depth of field distance behind that point will appear to be in sharp focus. That's kind of handy to know.
As you get to 'know' your camera you'll get a 'feel' for the depth of field you should be able to capture under different lighting conditions. So if you're shooting something say a person outdoors in decent lighting 8 - 10 ft away at 100 ISO, even at moderate apertures, with a 'normal' 30~50mm lens you'll get about 6 ft of depth of field, so 2 feet in front and 4 ft behind your subject will appear to be in focus. I say this because with macro work you'll be probably dealing with an apparent depth of field of barely more than 1/4". So what do you focus on ... well if it's a fish or invert, typically you want to focus on the eyes, as that tends to make a more dramatic human connection, as for other subjects are concerned that's where the 1/3 ... 2/3 rule comes into play, you'll have to mentally compose what you think you're going to get, and position your focusing point accordingly.
Now we know what the challenges are we can deal with them...
Light management 101, even the brightest tanks are just not that bright, photographically speaking. Then throw the macro problems on top of that and you can end up with a bunch of fuzzy looking tank inhabitants. You can raise your cameras' sensitivity to light by increasing the ISO setting, this comes with a price, noise (multicolored random snow), most digitals handle ISO 800 fairly well, and the 3 or 4 f/stops and/or shutter speeds you'll get will really help. Another option is to master the flash, it will allow you to increase your shutter speed as well as increasing your f/stop, giving you not only the ability to stop most motion, but increase your depth of field as well. As an added bonus it will overpower your tank lighting and virtually eliminate white balance issues.
I hope all this may help a little, I hope I didn't bore anyone, just want to share a little knowledge and help others become better at recording their love affairs with their mini-oceans.
A little practice and you can take shots like a pro ... Good Luck!
