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!