Alright, full tank. There's a lot of preparation in these shots. Before I even begin, I've typically scraped all the glass free of coralline and have pulled off my Mag-float off and set it aside. I also turn off all the lights in the room, and only shoot at night, to eliminate reflections from the glass. When the tank looks good (clear water, etc.), I line up the camera squared up with the center of the tank, then sacrifice a tiny bit of sharpness by bringing the camera up vertically just a few inches. This allows just an ever so slightly more "top-down" view of the corals, rather than actually looking UP at some of them. The camera is set to its widest zoom, so I can get as close as possible. In my case, this is about 4 feet away, so the 6-8" vertical adjustment is relatively not that large.
I set the camera on a tripod, and set the timer and mirror lockup (most people don't have mirror lockup, and I don't know how huge of a difference it really makes). I set the ISO to 100 (as low as it goes). I set the exposure to slightly underexpose (slightly too dim) by -2/3 EV. This helps avoid blown out highlights (especially SPS tips), as well as potential oversaturation problems.
Then I take several shots and use the one that has the least amount of motion in the moving corals, as well as the most visibility of the fish. I'm completely fine with blurry fish, that's just the way it goes. I also don't turn off powerheads, because it's not worth it to me.
Here's the RAW shot.
Next I crop the image down. Notice how it's actually not square. That will be resolved in the next step.
Using Photoshop's "Perspective" toolbox, I stretch the corners of the tank to touch the corners of the image. This way, the image is all square when I'm done. Here's where to find that toolbox:
And here it is afterward. Compare to the cropped shot to see the minute difference.
Next I'll do some color correction. This is fine tuning for white balance. The lights on my tank are too blue, in my opinion, but I try to match that anyway. Honestly, I shoot for realistic (that sounds funny after you see all I "alter" to keep the shot "real"), rather than ultra-cool.
The next step is to clean out all the debris and distractions. I'll use the Heal tool to wipe out all the specks in the water, stray snails, and stuff like that. My goal is to make the corals the subject of the photo, not all the junk. Without a doubt, the greatest amount of time is spent here. I'll scan through the entire image at full size (maybe 12x the size of my 19" monitor) and pick out all the specks and stuff. Also, the greatest liberty is taken here when I even remove equipment, such as the HOB filter tube and the skimmer pre-filter. The skimmer pre-filter is actually there, I just wipe out the two adjustment knobs and darken in the bright areas under the light.
Next step is fine tuning color. I'll select corals individually (I prefer QuickMask and the brush tool) and bring up their colors. Again, I'm honestly shooting for realism, not eBay-ism.
The last step is resize, sharpen and add a border for presentation. I typically display my images for the Web at 800 pixels on the long side. Width, in this case. For sharpening, I have upgraded to using "Smart Sharpen" instead of Unsharp Mask. In this case, I did a percentage of 500% with a radius of 0.1 pixels. Sometimes I'll bump it up to 0.2 pixels, and significantly drop the amount. I also adjust the type of sharpening from Gaussian Blur to Lens Blur.