If your run off EC is going up it's because your getting a nutrient build up in your media. Always test run off and adjust feed EC accordingly. So if your run off EC is coming out higher that your feed EC then lower your feed EC. If your run off EC is coming out lower than your feed EC then raise your feed EC.
That's the easiest way anyways, some of the more advanced growers keep feed EC at the same level and adjust the amount of feeds which in turn changes the amount of dryback in the media to raise or lower the EC. But that's beyond most of us mainly because it's difficult to test media EC and water content on coco. If you grow in Rockwood then growdan make a WC and EC monitor called growsense that halls you control drybacks to change media EC.
Hope that helps :)