All images © 2015 Steve Miller
For shooting clownfish, I like the 60mm macro lens on a crop frame DSLR, although this aspect could be achieved with the native lens on many of the better point and shoot cameras, or even a 100mm lens on either a full frame or crop frame DSLR. The lighting in a macro image like this is 100% artificial from the strobes, giving you full control without the variables of natural light. This makes things much simpler. There is also an advantage to having lots of light, i.e. dual strobes.
The easiest way to set up for these will be to use two strobes pulled in close to your lens. Then "stop down" the camera's aperture (increase in f/number) as far as you dare, and based on the power of your strobes. Hopefully this will be around f/16 to f/22, or least f/11, because you need the depth of field to be as large as possible. Depth of field is the range of distances from your camera's lens which are in focus, and it might be only an eighth of an inch. That means the center of the anemone is in focus, but not the front or back of it.
I use two powerful strobes (Ikelite DS160s), so I choose a low ISO between 100 and 160 and a shutter speed between 1/125 and 1/160 second.
Start out by wasting a frame. Take a shot and preview it to make sure the anemone is well lit and gorgeous. Now it’s a waiting game... these clownfish never stop moving so you will waste a lot of frames getting tail shots or heads turned. This is where a DSLR will really shine, because there is zero shutter lag. With a DSLR you are looking through a glass window—not at an LCD monitor—so when you pull the shutter it’s instant.
The clownfish will move in and out of focus depending on how far they are away from the lens. Your key focus point should be the eye. When previewing your work on these subjects, learn how to zoom in on it. If it isn’t sharp, you’ll hate yourself for swimming away. This can be a challenge because your depth of field Putting the focus on "spot" and keeping that spot on his eye is the best advice for capturing that critical focus point.
Sometimes having your lights come in from the sides can cause translucent animals like this to glow from the light passing through them.
The metadata (ISO, aperture, and shutter speed) are exactly the same on all of these clownfish (f/25 | 1/80th | ISO 100). All images taken with a Canon EOS 50D and dual DS160 strobes in TTL mode.