Fire beside water
Fire is direct and water is absorbent. Aries produces heat and expects it to clear the air; Pisces takes the heat in and carries it for days afterwards. That difference in how long an argument lasts internally is the single most useful thing to understand about the combination.
The complementary version is real. Pisces softens the edges of an Aries and makes it possible to say things Aries would otherwise leave unsaid. Aries gives a Pisces something firm to organise around, which most Pisces charts genuinely benefit from.
Cardinal and mutable
Aries initiates and Pisces adapts, which is a smoother modality fit than the elements suggest. Pisces does not compete for control, so the authority struggles that trouble Aries elsewhere mostly do not appear.
The problem is the opposite one. Pisces adapts so readily that Aries stops being told when something is wrong, and then discovers all of it at once, later, in a form that looks like it came from nowhere. Adaptation is not agreement, and the pairing works better when both parties treat it as separate.
Mars, Jupiter and Neptune
Aries is ruled by Mars, the most straightforward body in the scheme. Pisces answers to Jupiter traditionally and Neptune in modern practice, neither of which is straightforward at all.
Mars against Jupiter tends to be generous and prone to overreach. Mars against Neptune tends towards idealisation, and idealisation is the specific risk here, since it is comfortable for a while and then collapses in a single afternoon. The actual conditions of those planets, not the rulership list, determine which applies.
Where it grinds
Honesty about dissatisfaction. Pisces avoids the confrontation and Aries needs it to happen in order to know anything is wrong. This produces a relationship in which one person believes matters are settled and the other has been unhappy for months.
The second grind is bluntness. Aries says the true thing quickly and considers that a courtesy. Pisces registers the delivery as much as the content and remembers it well after Aries has forgotten the exchange took place.
Why two suns cannot settle this
A sun sign describes a tendency spread across everyone born in the same few weeks. Two Aries and Pisces couples can look nothing alike, because the rest of the charts differ entirely.
Moon against Moon tells you whether the emotional pace is compatible, which is the crux in this particular combination. Venus tells you what each expects affection to look like, and Mars tells you whether one person's approach to conflict is survivable by the other. Read those before the suns.
Questions people ask about this
Is Aries too blunt for Pisces?
Often, though the more common failure is the reverse: Pisces not saying it is a problem until a great deal has accumulated.
Does Pisces give in too easily here?
Pisces adapts by nature, which Aries can mistake for agreement. Treating adaptation and agreement as different things helps considerably.
Who rules Pisces, Jupiter or Neptune?
Jupiter in traditional practice and Neptune in modern. Both are worth reading, since one describes expansion and the other describes idealisation.
What matters more than the sun signs?
The two Moons above all in this pairing, then Venus and Mars. Emotional pace is the crux and the Moon governs it.