The transparent fish in the photos below probably lacks black pigment, called "nacre". It's exact species is unknown but my guess is that is an X-ray fish - Pristella maxillaris.
Cropped photo of freshwater X-ray fish
Photo of freshwater X-ray fish
Photo of freshwater X-ray fish
For more hi-resolution photos, check out my Flickr Fish Gallery.
Photo taken at Sarawak Museum Aquarium.
Other type of transparent fish: Zebrafish
Some transparent fish such as the see-through zebrafish are use by scientists and researchers to directly view its internal organs, and observe processes like tumor metastasis and blood production after bone-marrow transplant in a living organism.
Other type of transparent fish: icefishes
The icefishes (or white-blooded fishes) are a family (Channichthyidae) of perciform fish found in the cold waters around Antarctica and southern South America.
Their blood is transparent because they have no hemoglobin and/or only defunct erythrocytes. Their metabolism relies only on the oxygen dissolved in the liquid blood, which is believed to be absorbed directly through the skin from the water. This works because water can dissolve the most oxygen when it is coldest. In five species, the gene for myoglobin in the muscles has also vanished, leaving them with white instead of pink hearts.
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* Added May 26, 2008: Photo of Pristella maxillaris - X-ray fish - Series #2
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