Fabric pots are the best addition too. Oldschool pots suckIt sounds like the plants aren't drinking and/or pots aren't draining. Like others have suggested, use H2O2 to kill and rot in the medium. I'd give them a good flush of half-strength nutes + H2O2, and make sure they are really draining well. I never liked straight coco myself because I felt it didn't drain as well as it did once I started adding perlite, however many straight coco users will swear that straight coco is the best. Can you post whole plant pics?