The overall drawings are perfectly outlined, well bolstered, and your coloring is fantastic. A tip you could learn however, to make the appearance of the Koi explode and beautify, is to shade its fin's and tail, as well as mark small, detailed areas on the scales and eyes.
To shade, all you have to do is pick your color of choice. Recognize where you want the shadow of the image to begin, and the light of the image to end. For instance, if that red Koi is pointing down, the bottom-areas of it would be dark, and the top areas would be lighter than the darker colors at the bottom. Shading is all about shadow perspectives. To shade with tools such as pencils, crayons (I can do it with markers, spraypaint and pens though.), drag your color along, pull off lighter, and then completely pull off once the color seems to "evaporate" into the white of the page. :)
This should help you nicely.