I really appreciate it how you chose a subdued but dark red color when making the giant koi fish and the samurai. Otherwise if you had used a bright red, it would have looked out of place and awkward. I like the dark blue/green colors that you used for the water, and how you chose a lighter color for the leaves on top of the water. You did a very good job at making everything blend together.
It's like I'm out with my grandfather at a very serene and tranquil Japanese garden with a little pond. It is there that my grandfather tells me a story of a very valiant samurai that went off by himself to go fight a great evil water-demon that was causing havoc and chaos for people. He rode off on a giant koi fish. But he never came back. But soon after things got better. It's like you made this picture in the middle of when my grandfather was telling me about that legend.
Whatever story is behind it; it's like this picture was done in remembrance of a legendary/ mythical samurai warrior. It's a very nice picture. Very nicely colored.