I have been running a Korallin S-1502 BioDenitrator for a year now, and it still has not reduced the NO3 level at all. I followed the instructions exactly. However, every once in a while, I will find that the top of it is filled with air. I think that there may be too many bubbles in my sump at times, and the feeder pump pumps them into the denitrator. Anyway, now the sulpher looks dirty and brown. Should I try to clean it? How do I do so?