This reminds me of the time I've tried drawing a mermaid. I was just learning how to work with anatomy in the picture at the time and had some trouble figuring out the length of the arms and tail. It was one the first attempts at drawing people and it came out well, except I made the arms too skinny and the tail too long for the mermaid's age.
Its not easy working out of your comfort zone, but I'd say you've done a great job. The colors and design are beautiful. I will admit that the only thing I think needs work are the tails. To me, one looks a little too short and the other a bit too long, considering where they're suppose to bend. But don't be hard on yourself, those type of tails are hard to do because of how human legs look. You could try this trick where you indicate where a character is standing with their feet together. The tail will be from the character's hips to their ankles, and then draw the fins right where the feet are suppose to be. Or find an outline drawing of a character standing in that position and, with another color pencil, practice drawing a fish tail on them. That way you can work with the curves of the figure. Give it a try and see if it helps.
Other than that, I think you've made a great start.
(Sorry for posting this more than once. I've only just figured out how to write a critique on DA.)