Aquarius probably makes the best ex. They would never fuss about losing a husband or a girlfriend because it is you they love, not the function you perform in their life. When your relationship ends, you will still mean a lot to the Water Bearer you once loved.
The problem that Aquarius may cause is not sulking or stalking, it is the friendliness they still demonstrate. Your current partner may be dissatisfied with your ex always being around as if nothing has happened. To avoid misunderstanding, make sure you limit your interactions with Aquarius to a decent amount and talk it out with your current partner.