I disagree that this is unnatural. To be honest, it's not as if it's that easy to generate something like this artificially. Well it's possible, but it'd be more believable if it's exactly as SA says - it's a genetic mutation.
Which happens all the time, and if someone breed as many Clownfish as SA, they're bound to find this, or something else caused by a random mutation eventually.
It's actually very natural for this to occur.
Well yes, I guess it's possible that such a mutation can cause issues, perhaps by making them swim slower - resulting in either being outcompeted, or killed by predators.
Yes, it's not natural for it to survive, I guess. Lucky for this one, it was breed in a glass tank.
And well, we all keep fish in glass boxes so... can't really say anything about it. @_@ If we want it to be natural we better all toss Lionfish into our fish tanks, and let them eat up our fish. Because that's what's happening on the coast.

(Of America)