If it's coco and not some weird mix. My favorite watering method is to water through. Every day I feed with full nutrients. But I water through. So I flush and feed at the same time.
I water till about %20 of what I put in, comes out. This flushes all the old crap out and replaces it with perfect ppm. It takes more nutrients, But you know exactly what's in that coco.
If you don't water through or flush every now and then, your unused nutrients stay in your medium, and build up.
If you're using unmixed coco, just flush it with regular water and it should fix the problem. The damage won't be reversed, but new growth should be fine.