Saturn rules Aquarius, it is the sole domicile ruler in traditional astrology.
In traditional astrology, you do have other rulers of the sign -
the triplicity rulers, bound rulers and decan rulers.
Triplicity rulers of Aquarius are Saturn, Mercury and Jupiter.
Bound rulers depend upon the degree placement, you need the egyptian bounds table
to determine based on the degree.
Decan or Face rulers are based on three division of the sign.
The domicile ruler is the most important, +5 score.
Next comes the exaltation ruler, +4 score. Aquarius don't have exaltation ruler.
Next is triplicity rulers (+3).
I think the original post is concerned only with domicile ruler. So, yes Saturn is the sole domicile ruler.