Could be a reflection in the water. where did you find the pic? Could also be dyed. The dyed fish busines has started dying loaches now. I say pictures of some "berry" colored skunk loaches the other day. Y-U-C-K!!!!!!
This REALLY looks like a computer graphics job. I studied graphic design for 2 years and I am 99% sure this picture was altered in some photo imaging program like Adobe Photoshop for example. To make a loach this green is VERY easy in a photo imaging program.
If you save this picture and zoom in a lot, you'll see that around the contour lines of the loach there is green color sticking out. This means someone carefully painted the loach green in a photo imaging program. Believe me
If this is not the case, the loach was definitely dyed or something of the sort, because in nature you don't have such green loaches. (But I am really convinced the photo was altered - that's all.)
LOOK HERE: (you should see green color sticking out beyond the fin)