I spent most of last night going through text books and old photo logs, to come up with examples of Magnification being the main control of DoF. I have 20 or so actual examples, But all were 4X5 sheet film images where the subject was smaller than the image captured. Things like a needle (stylus) from a turntable. A dimond ring, and a industrial gear box.
The information I have supplied above with the equations and formulas still do apply to all macro photography, but become more evident when image size is greater than subject size.
To redirect the thread this is a list of what I feel are the most important things to check when shooting you tank
1) turn off actinics to allow your camera to white balance on 1 colour temp
2) test white balance to find the best area. Or use white card to get correct white balance
3) use spot metering and meter on your subject
4) clean your glass (no one wants to see algae or coraline
)
5) if you can not get an exposure over 1/125 use a tripod
6) use a light box if you have access to one
7) do not use flash
Jason