I'm not totally sure, but the best match on Fishbase looks to be the pearlscale butterflyfish, Chaetodon xanthurus (plus quite a lot of chromatic aberration).
Forgot to change the name and now came by accident along a species which looks exactly like the one one the picture, however not called Pearlscale butterflyfish Cheatodon xanthurus but Seychelles butterflyfish C. madagaskariensis.
Any specialists which know the difference and which of those is on the picture ?
I'm no expert but the most obvious differences on comparing photographs are that the stripes across the body on C. madagaskariensis do not cross, and the eye stripe doesn't extend as far superiorly. There may be more obvious features but that seems straightforward enough to distinguish the two.
