That is amazing, really nice, not static, it looks like it is swimming. Your first? Clearly you have talent, I hope you keep at it and I look forward to seeing your hundredth...if it doesn't swim off the canvas.
I have been doing taxidermy as a part time/hobbyist since the 70's .... so I have painted plenty of fish (skin mounts & reproductions). I just never did any flat art.
Here is a cutthroat trout (reproduction) that I painted for a customer. The fish blank starts out white, then I start adding color with an air brush, dry pigment powders, etc ....