I saw some black in the photo of the screen and I wasn't sure. When my flow was to slow, I got some black specks and I consider it an indicator of not enough flow. I'm pretty sure the specs were dead bacteria, it also got a musty smell in the reactor. I would raise your flow anyway if you can, that might help keep your screens cleaner. I don't get any build up in my screens like your getting, could be differences in set up, and for that matter, reactor. Just an idea.