My research started with looking at common pond fish - the search came up with fish like Goldfish, Koi, Ghost Koi, Common Carp, Sturgeon, Catfish, etc.
I really like the large mouth of the Catfish
The Goldfish is beautiful, but with some subtle added features it could look beautiful yet deadly
I found the spiky fin of the Perch beautifully dangerous.
I then remembered a fish that my brother had in his fish tank a few years back. The Black Moor Goldfish is perfect for the type character I need because it ticks all the boxes.
There appears to be a slight lid on those massive eyes, which gives me the ability to make an angry frown. I also like how like how the overall shape of the fish is a triangle, yet the body is still very round, giving an evil yet playful character.
This was a very fun character to draw, the large eyes made it fun to watch them come alive, and the agile body helped make a fluid motion in the drawing.
Here is an initial turnaround which as the character develops, so will the turnaround.