I did a water change/gravel vac last night and tonight, in my 37 gallon and my 46 gallon tanks. My 37 gallon houses 7 african cichlids and a synodontis catfish, the filter is barely adequate (Aquaclear 30), and the water wasn't very dirty. My 46 gallon houses 13 fish, including 2 clown loaches, 1 common plec and 2 cories, and the water that came out was very dirty (although before I started it looked very clear). I have an aquaclear 70 on the 46 gallon (larger than adequate). How do you explain that?