Just give it time. Assuming you’re gonna flush for week or two, maybe every two-three days or something do a flush like you did today and eventually it will come down to the lower hundreds.. I’m a coco guy personally and that’s why I like it.. it doesn’t hold on to nutrients. If you run a few gallons thru it, it will leech 90% of the salts left in the medium.
Anyways.. give it some time and monitor that run off. You’ll be good