According to my data, these are the breeds that give twin-fin variants:
Twin-Fin + Twin-Fin (big suprise there)
To be honest, the twin-fin and the spotanus both appear to be types that cannot be bred without having the original fish/fin first. A speckled+flagged or speckled+finless would both yield a greenfin if you didn't want to wait for a twin-fin.
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