Welcome to pixelation, Richard! Congratulations on moving away from white backgrounds and black outlines, you have taken the first big step! Some thoughts:
1 - My first impression is that the beast is quite flat - it looks more like a fossil or a relief than a really 3d animal. Try checking out photos or paintings of fish and see how the lights and shadows work on them!
2 - The anatomy is pretty confusing here. The tail is very reptilian, not fish-like at all, and the head and foot/flipper construction is rather hard to read. Do you have a reference for this creature? That would help us to focus our feedback more. New pixelers and new artists in general will sometimes be tempted to draw fantasy creatures simply so they don't have to learn how to draw real creatures...but you'll find it is very hard to draw good creatures, even if they're imaginary, if you don't have a grasp of the fundamentals of existing beasts.
3 - Your animal is a pretty uniform, light color all over, which makes it
a little boring to look at.
Be sure to check out
tutorials thread, where you will find all sorts of good pixel and general art tutorials!