My xp3 started to leak very slowly a few months back but it stopped and haven't had a prob since priming is a breeze. Was there slack in the intake and out take hoses? That's the only thing i can think of that would prevent it from priming properly. I like the water flow from the xp3 compared to the fluval 405 both are running on a 125 gal but when i replace the xp3 i will be getting a fluval fx5.