How do you know?

Another fish, the cleaner wrasse, can apparently recognise itself in the mirror, hinting at some for of self-awareness.

https://www.nationalgeographic.com/animals/article/fish-clea...

A being is conscious if and only if it feels like something to be that being. What you're referring to is self-consciousness. Goldfish are most surely conscious but not self-conscious.

Stating that I can imagine X to be Y is not a statement that X is Y, only that if X is Y then nothing changes about my model of either X or Y.

That said, I'm not able to verify from a brief search for consciousness and self-awareness that the two are synonyms. The two seem to be related but are treated differently from one another. Furthermore it's not even clear whether goldfish are conscious or self-aware. Seems like it's an open question.