Spoke to Mark, he also doesn't think it's a Loodvis.

I reckon it could be a Chiselmouth, judging by the size and shape of the body, though they are normally a little more yellow/golden in colour that can also be explained by the coffee colour of the water as in the pictures.

Think of how black clearwater Bass can be as compared to how silver they get when living in discoloured waters.

If the OP can tell us the river or trib, we could get a better idea of whether there are LS or Chiselmouth in it.