The blue ribbon eel is a fairly common inhabitant of reefs in the Indo Pacific region. In Indonesia, they are found in places such as Bali, Sulawesi, and Ambon. These skinny eels are incredibly colourful and make beautiful photographic subjects. However, many photographers often make a mistake while photographing them. The mistake is using a high f-stop such as f16 in order to get as much of the eel in focus as possible. What this technique creates is a distracting background that is too much in focus. As the eels are usually found with their heads sticking out from their home hole, they are the perfect subject to work with when it comes to isolating the subject from the background. There are several ways to isolate the subject from the background such as using a snoot for a black water background or using the technique in this photo.
This particular photograph uses the limited depth of field technique with an f-stop of f5. The limited depth of field creates a blurred background which leaves only the eyes of the eel in sharp focus. Due to the bright colours of the eel, the creature stands out from the more monotone blurred reef. Black backgrounds are another very popular way to get the subject to stand out from the background.
A fast shutter speed is typically desired for this style of photograph, to control the natural light. Set strobes on TTL or else on a very low power setting.
If you would like to learn a little more about this style of photography have a look at Shooting Limited Depth of Field. You can also read about it here for a more general understanding of Limited Depth of Field