I have a slightly overpopulated 29 gallon aquarium that has a continuous small problem with black algae on some of the rocks and artificial plants. My bigger problem is inside of my filter, a Fluval C-4. The sponge section and the small pad on top of the bio media become clogged with black slimy material on a weekly basis that won't wash out and forcing me to change the pads. My water parameters are great, 0 ammonia, 0 nitrates, 15- 20 nitrates, pH 7.6, phosphates <0.5 and the GH is around 80. I'm using the standard sponge filter in the C-4 along with the standard bio media. In the chemical compartment I have a bag of SeaGel and a bag of Purigen. I'm also running a small 7 watt UV sterilizer the filter and a 24" bubble wall along the back of the tank. I'm assuming the material on getting inside of the filter is more of the black algae but I'm not quite sure. Does anyone have any ideas what the material in the filter is and what I can do to prevent the black algae buildup in the aquarium?