At least from my experience, a familiar is a spirit which helps you in your craft. Many familiars are naturally atracted to magic and they don't even have to have a corporeal form. As long as you somehow incorporate your fish into your rituals (take it inside your circle, use some water from it's bowl etc.) and it doesn't seem distressed or it doesn't interrupt the ritual somehow, I would consider it a familiar. https://www.youtube.com/watch?v=0mwhvQ2TWjM (Excuse my English)
At least from my experience, a familiar is a spirit which helps you in your craft. Many familiars are naturally atracted to magic and they don't even have to have a corporeal form. As long as you somehow incorporate your fish into your rituals (take it inside your circle, use some water from it's bowl etc.) and it doesn't seem distressed or it doesn't interrupt the ritual somehow, I would consider it a familiar. https://www.youtube.com/watch?v=0mwhvQ2TWjM (Excuse my English)