A couple things...first, you could try about a saucepot or so full or rice hulls to see if that improves your runoff speed. Also, +1 on trying to replace the standard vinyl tube. Maybe try some of the reinforced braided PVC tube you can get at HD or Lowes. Lastly, you'll probably want to solve your runoff issue before going into HERMS since HERMS relies on a certain runoff speed. Good luck!