I have two gulper cats in a 55 gal, and they are both still small, and Ive done 50% water changes for the past 3 days but my nitrate levels are still at 40 ppm. This is ridiculous. I have two AC 50's on the tank. How often should I clean out the filter sponges? One of them is pretty gunky, but I only setup this tank about 2 weeks ago. If the sponge gets filled with too much crap will it start producing more nitrates?