I find it interesting what I will say in front of acquaintances and what I won't. Often enough I avoid things that require extra explanation when I don't want to get into it.

"Goddaughter" is pretty well understood. The young person whose moral and spiritual education I have the honor and responsibility of guiding. Not actually biological.

"Fishdaughter" is really nonstandard. Why fish? What is entailed in a fish relationship? How did you come by your responsibilities?

Sometimes I will explain: book club, internet, accidentally a whole family. (Is that bad?) Sometimes I say "goddaughter", and let people make assumptions that are not all that far off.