Be sure to move any decorations and rocks you have when vacuuming. Also shake out any moss you have. You would be surprised how much stuff gets pushed under these things or in the moss. I had problems with ammonia a while back and took almost everything out of my tank and vacuumed it. It was unbelievable how much stuff was under the rocks and seashells in my tank. Plus, when I shook out my moss "tree", it was like a snow storm had occurred in my tank. All kinds of crude was stuck in it. You also want to stir the top layer of sand to release any dead bga.