Cutie marks are based on the fish's personality, color and or name for example I gave Morgan a skull and crossbones because he's named after a pirate and Igneel fire because he's named after a Dragon and he's fire colored with a fiery personality
With that in mind I'll need some details about your fish,
Eye Color: Really light-colored, maybe blue? It's difficult for me to tell, curse you color-blindness! Ringed in black.
Personality traits: Curious, friendly, a bit skittish, always hungry, playful, likes to act tough, but then turns tail and swims away... a goofball. XD