Air over water
Air separates itself from a feeling in order to examine it. Water does not perform that operation at all, and Pisces least of any sign. So Aquarius describes an emotional situation in the third person while Pisces is already inside it.
Each finds the other's approach unnerving. Pisces reads the detachment as coldness, which it is not, and Aquarius reads the immersion as a loss of judgement, which it sometimes is. The useful outcome is that each borrows a little of the other's distance setting.
Fixed and mutable
Aquarius holds; Pisces adapts. In practice this means Pisces accommodates, repeatedly and without announcing it, until an accumulated grievance surfaces from nowhere and Aquarius has no idea what it refers to.
Mutable signs rarely fight at the moment of the offence, and fixed signs assume that silence is agreement. That combination produces disputes that are years old by the time they are spoken aloud, which is the most reliably predictable thing about this pairing. The repair is unglamorous: Pisces has to raise the objection at the time, and Aquarius has to stop treating silence as data.
Uranus and Neptune, with Saturn and Jupiter behind them
Modern rulerships give Aquarius to Uranus and Pisces to Neptune, two slow bodies that sit in the same sign for whole cohorts. That is worth remembering when anyone claims either sign is a generation's defining temperament.
The traditional rulers are more personal in effect. Saturn behind Aquarius insists on a structure; Jupiter behind Pisces keeps expanding past it. Where those two sit, and whether they aspect each other across the charts, matters more than the outer pair.
Where it grinds
Boundaries, in both directions. Aquarius sets them and defends them intellectually. Pisces treats them as provisional and crosses them affectionately, which reads to Aquarius as a failure to listen rather than as warmth.
The other friction is over principle against person. Aquarius will follow a position to its conclusion even when someone present is hurt by it. Pisces abandons positions readily to spare somebody, and considers the willingness to do so a virtue. Neither view survives contact with the other undamaged. What usually happens is that both quietly moderate, and neither admits to having done so.
What actually decides it
Not the Sun. A sun-sign match summarises a huge population and says nothing certain about any two members of it, and the further into a real relationship you go the less of the summary survives.
Moon contacts govern whether the two feel safe in the same room. Venus governs how affection is offered and recognised, and Mars governs the arguments. Comparing those across two charts is a different exercise from comparing two Suns.
Questions people ask about this
Do Aquarius and Pisces have anything in common?
A tolerance for the unusual, arrived at from opposite directions. Aquarius reaches it by reasoning and Pisces by not filtering, which looks similar from outside and is not.
Why does Pisces feel unheard here?
Because mutable signs adjust quietly and fixed signs read quiet as consent. The accommodation is real and largely invisible until it stops.
Are the outer planet rulers useful?
Only partly. Uranus and Neptune move slowly enough to describe a generation, so the traditional rulers, Saturn and Jupiter, give more individual information.
What matters more than the sun signs?
Moon, Venus and Mars, and how they aspect across the two charts. The Sun sets a broad tone and the others describe the daily experience.