There aren't enough classical painters uploading content in this digital age, very nicely done with perfect color choices to really capture the koi look.

Hey pretty nice fish; I like their gesture swimming upstream!;
About the colors; I'm not too experienced on traditional painting; but in my experience the black paint/ink gives some trouble working among other colors; sometimes it feels like it 'absorbs' the vibrance of the colors surrounding it; like in the scales of the orange fish, which don't look as 'bright' as they could (on the other hand; the effect works better on its tail since it's more balanced) it may helps to find a mid-tone for the darker spots.
That's what comes to mind; keep it up!

Oh wow, this is really pretty! Well drawn :O

Wow,there's so much 'life' in this picture like something inside a book or art museum and the colours even match those on the real fishes.Well done and keep it up,miss!

Oh and it's called 'symbol',a little mistake you made there.In some Asian cultures,these fishes represent wealth and beauty too.