It is MUCH easier to tell when they are in water than in the air, because the colours look different.
I would think that it's some kind of "starlight bristlenose", my best guess would be
L182
Ohter possibilities:
= L183
L184
or perhaps (less likely):
This is a link that will give you thumbnails for all ancistrus species...
If none of the first suggestions seem a good match, maybe you could get a photo of the fish under water, where it's more easy to see it's true colours.
[Also, if you could crop the picture down to maybe 600 pixels wide, it fits better on most peoples screens]
As Yann mentioned, there are MANY species that look almost the same...
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Mats