For 4 years I have aerated my 1 acre pond with easypro scr50 1/2hp pump with 400ft 5/8” poly line going to my pond then at the pond I have a splitter to two double headed diffusers. The line from the splitter is 1/2” weighted for about 60 ft to each diffuser. My problem is the after 4 yrs I have replaced the gaskets, cylinders of the pump because the pressure dropped to the point I was getting nothing from the diffusers. I also cleaned the 4 diffusers at this time. I have walked the entire length of the line from the pump to the splitter checking for air leaks. I have checked the check valves on the diffuser plate and all looks great. I can put the diffusers in 2 ft water and they bubble away but when I put them at 6 ft water I get nothing. The PSI at the pump is about 20 PSI which is suppose to be 3.7CFM. I am getting what I believe is great air pressure at the splitter. Any ideas why I get nothing at 7ft but get 2ft diffusers bubble away?