I always thought it was Taurus and Pisces to be honest with you.
But Earth sign and Water sign could go to together. I just find that if the Pisces and Cap are both over achievers, it can get ugly, plus Caps aren't known for sensitivity, and Pisces seem to have a tough outer shell with a really fluffy soft inner core.
So I dunno on this one, I would rate them about a 70% compatible . . .