Beautiful tones. Can't tell if your focus is on or it's just the translucency of the subject, so I'll give you that one. Is this your tank or at an aquarium?
TC
The composition is wonderful and very nice to look at, but it doesn't seem to pop out. Increasing the contrast would definitely benefit this shot. Getting that dark background blacker would help.
Excellent idea, excellent capture. Good composition and lighting. There is some noise, I imagine from high iso - but this image is good enough to overcome that, and probably hard to avoid. :-)