The colors on these are superb. And in terms of anatomy, form, and lighting, spot on.
The character designs, though, are ok but are lacking in defining features. You said they were fish girls, but I wouldn't have known that had you not mentioned it. I'd suggest adding some fishy designs on their clothes or gills to make them more unique.
The girl on the right is great--proportions are well-balanced. The girl on the left, though could use a more elegant/realistic/flowing pose. I get you were trying to make the whole thing curve like a fish but something is off and it feels a bit stiff. The lips are huge but I think there needs to be something else that is bloated towards to bottom maybe or behind the head to balance it out. Goes back to not knowing they're fish girls at first glance.
Just my 2 cents.