Matches with Pisces
Other Water signs seem the obvious choice here. But Scorpio might overpower Pisces, and Cancer might be too clingy. The signs sextile to Pisces are Capricorn and Taurus. While Capricorn might be too limited and grounded for the Piscean imagination, Taurus probably fits right in. Gemini, also, because it's a mutable sign like Pisces, can be compatible.
The real shocker here is that Sagittarius may just be the best combination for Pisces. Though they're such opposites, they complement each other quite well. Pisces is able to soothe the Sagittarius savage beast — and they fit like two pieces of a puzzle. Pisces also lets Sagittarius do what he wants, yet always keeps the upper hand with a cool, polished, quietly strong demeanor. And this is what Sagittarius likes best. As for the other Fire signs, there doesn't seem to be much chance, but it truly depends on the other factors in the two separate charts.