Black sludge on gravel


Hi, have a large pond(15000 gal) with a gravel type bio filter about 5ftx10ft x 2ft deep.My question is what is the black sludge that is in the gravel.Has been in operation 3 years and I flush the filter letting the water run into the grass.Is the black stuff bad or not,thanks,Richard


The black “stuff” is accumulated organic waste. It is not in and off itself bad, but will clog the filter and deminish its effectiveness over time. The sludge (black stuff) accumulates because there is not enough oxygen to cultivate an adequate colony of beneficial bacteria to digest it. Adding an air pump and some beneficial bacteria will help to remedy that situation.