Air and earth, working at different scales
Air reasons in the abstract; earth works with what is already standing. Aquarius arrives with a model of how a thing ought to operate, and Capricorn wants to know what it costs, who administers it, and how long before it pays for itself.
That is not hostility. It is two halves of a competent process. The version that fails is Capricorn dismissing the model as theory and Aquarius dismissing the costing as timidity, which leaves nobody doing the other half. The pairing is at its best when each accepts that the other is checking something real.
Fixed against cardinal
Modality tells you more here than element does. Capricorn is cardinal and initiates; Aquarius is fixed and entrenches. Capricorn will move first, and Aquarius will decline to move at all once a position has been reasoned out.
The consequence is a standing imbalance in who begins things. Capricorn ends up carrying the starting energy of the pair, and Aquarius supplies durability. Both parties eventually resent the arrangement they have created, usually without naming it. Naming it tends to help, and neither sign volunteers that conversation.
Saturn on both sides, Uranus on one
Traditional practice gives Aquarius to Saturn, modern practice to Uranus, and Capricorn belongs to Saturn either way. So the pairing puts restraint, patience and long time horizons on both sides of the table, which is why these two often understand each other's seriousness immediately.
Uranus is the difference. It introduces a willingness to break something functional in order to replace it with something better, and Capricorn has no equivalent impulse. Where Uranus sits, and how Saturn is placed in each chart, decides how sharp that split gets.
Where it grinds
Warmth. Neither sign volunteers it, and neither reads its absence as a problem until quite late. Reserve compounds across a pairing like this, and a long stretch can pass in which the arrangement functions and nobody is enjoying it.
The second grind is status. Capricorn takes hierarchy seriously as a working fact; Aquarius treats it as an arbitrary convention worth ignoring. That disagreement surfaces at other people's weddings, in front of other people's parents, and in how each behaves at work.
Why the sun signs settle nothing
A sun-sign pairing is a statement about a very large group of people. It describes a tendency in the aggregate and cannot describe two individuals, and anyone reading this who recognises none of it is not the exception that disproves it.
The Moon governs what each person needs domestically, Venus how affection is offered, Mars how conflict is conducted. Those three usually do more work than the Sun, and comparing them across two real charts is what changes the answer.
Questions people ask about this
Are Aquarius and Capricorn too similar?
They share Saturn and therefore share a tolerance for delay and difficulty. What they do not share is the reason for building anything, and that difference keeps the pairing from being redundant.
Which one gives way in an argument?
Usually Capricorn, on tactics, because cardinal signs redirect more readily than fixed ones. Aquarius rarely concedes a principle, which makes the concessions look uneven over time.
Does the shared Saturn rulership help?
It gives both a similar attitude to time and obligation, which removes one common source of conflict. It does nothing for the warmth question, which is the harder one here.
What should I read instead of sun signs?
Moon, Venus and Mars in both charts, and the aspects between them. Those describe daily life together far more accurately than the two Sun positions do.