Have you taken the pump apart to see if it's all gunked up on the inside? I had a pump in my rez for a while before it burned out and I'm pretty sure it was jammed up with yucca and kelp. I've heard that sometimes the metals inside the pump can get magnetized and kinda screw with the metals in the feed like the iron and the copper and stuff and if that stuff is sticking to the pump it could lower your ppm
