What it does to a planet
Aquarius abstracts. A body here steps back from the immediate situation and evaluates it against a general rule, which produces impressive consistency and a certain coolness in the moment. The Moon in Aquarius handles feeling by analysing it from outside; Venus in Aquarius needs friendship before anything else; Mars in Aquarius fights for a position rather than for itself.
Fixed, not chaotic
Aquarius is often described as unpredictable, which contradicts its modality. It is a fixed sign, and fixed signs hold. What is unpredictable is which position it holds, since the sign chooses on principle rather than convention. Once chosen, it does not move, which is the opposite of erratic.
Two rulers, usefully disagreeing
Traditionally Saturn rules Aquarius; modern practice assigns Uranus. The pair describes the sign well. Saturn accounts for the fixity, the discipline and the long adherence to a structure; Uranus accounts for the willingness to break with one. Reading both, and noticing which is better placed in the chart, resolves most Aquarius placements that seem contradictory.
The misreading
Aquarius is described as humanitarian, which is true in the abstract and often misleading in person. The sign cares about people in general with real conviction and can be notably impatient with the specific one in front of it.
Leo opposite is where the correction sits: the individual case, the personal warmth, the willingness to be involved rather than correct.
Aquarius, and the generation problem
Uranus spends about seven years in a sign, so its Aquarius transit belongs to a cohort rather than a person. That matters for reading Aquarius placements: the sign itself is personal when the Sun, Moon, Mercury, Venus or Mars occupies it, and considerably less so when it is Uranus, Neptune or Pluto.
The practical rule is to check which body is actually in Aquarius before drawing conclusions. An Aquarius Moon is a specific and unusual temperament; Uranus in Aquarius is shared by everyone born between the mid-nineties and the early two-thousands. Descriptions that do not distinguish the two are describing nothing in particular.
Questions people ask about this
Is Aquarius ruled by Saturn or Uranus?
Traditionally Saturn, in modern practice Uranus. Reading both explains the sign better than either alone.
Why is a fixed sign called unpredictable?
Because it chooses its position on principle rather than convention. The choice surprises people; the commitment to it does not waver.
Is Aquarius a water sign?
No, despite the water carrier imagery. It is an air sign, and it works through ideas rather than feeling.