First thing I see are that you have a lot of different shades of blue. By my count you have 6, and two of them are almost indistinguishable. It's a small piece so you might not need that many. It also looks like there aren't many dark colors. There's black in the eyes, but the darkest the brown and blue get are only moderately dark. Not as much contrast between the colors that way. That's fine if that's what you're going for, but if you want less contrast you would probably want to cut down on the number of colors then.
Some of the outlines, like the spot on the cheek-fin, give it an odd kind of shape. The outline isn't really straight there and it gives that particular spiny part a little wobbly looking. You could lighten it to give it more of a pointed look. It's probably fine to lighten the outline there so much because there's a dark grey behind it which will keep the light color from bleeding off the edge of the image.
I guess the other thing that I notice is that there are come places where you have two colors side-to-side where they're very close in how dark they are. Look at the mouth area on the right side where you have a medium grey outlining a dark-ish pink. The way I think of outlines is that they should be darker than whatever they're outlining, generally.