I never waste time on stuff that won't be seen. A lot of folks enjoy that "but I know it's there" feeling, but it does nothing for me! Heck, I don't even finish the bottoms of armored vehicles, unless it's on it's side.
But, if I can see something through a window, canopy, or hatch, I'd probably give it a go.
As far as visable detail goes, I just start with the obvious things and work my way down to as far as my patience will take me.
I think for most of us it is much more a matter of patience rather than skill.
Oh, and I don't think anyone answered this: A
koi pond is a nicely landscaped pond for koi, which are big, colorful carp. The word
koi is Japanese, and you can find puh-lenty of koi ponds here. I'm not sure, but like many thngs in Japanese culture, it may be a custom that comes from China.