Well this is what I've been doing.
Fill my res and add canna a&b etc and measure my ec with a Bluelabs Truncheon. Usually no higher than 1.2.
Then feed it to my plants.
That's it.
Should I periodically flush a load of water through them?
Plant's have no problems it just whether It's required to flush the salts and buildup of nutes from the roots.
From what I understand this would remove the high levels of nitrogen etc which is causing black marks around my spliff, and a harsh smoke.
I've never measured any run off etc....hmmm.