Black particles in biopellet reactor?

[razzpatazzz]

I've been getting this blackened particle along with a rotting smell in my biopellet reactor ever since I cleaned it out a while ago. I tried to look it up and most people seemed to think it was a slow flow causing a build up of hydrogen sulfide. I slowly increased flow to max over a few weeks and the smell definitely went away,  but the black particle is still all over the reactor and nitrates have not been going down. Any tips on what might be causing this and how to fix it? 

[Origami]

The rotting smell, if it smelled like rotten eggs, was likely hydrogen sulfide. If it didn't smell that way, it wasn't. The black stuff looks like filaments, not particles. It also looks like it's growing on the walls of the reactor, not on the pellets. Could it be an algae or bacteria growth? Why not just take the reactor offline and clean it?

 

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