Looks very nice, but I think the koi lacks flow in his movement. I think it may be due partly to scales, they don't follow the move in the entire end part. Also the scales get bigger in that part compared to towards the end in the front part.
Maybe you would need a little shading there too, to emphasise the flow, but I'm not sure. But definitely you would need to correct the scales.
To me it looks like you didn't think of how the scales actually look when the fish turns his body like this. I'm not into koi stuff, but I know there are lots and lots on YouTube about how to draw kois (really it's the same whatever you draw. You have to think of the object as a whole, not separate parts. What you don't see on the fish still affects how it looks.)