Last pic for comparison was taken just over a month ago.
I cleaned Oliver's sponge filter today. It rarely needs it- one fish and a snail population don't seem to create much waste in here to clog it up. But I did notice there was a lot of brown mulm in the base where I have bio-cubes. Figured a long time ago, if I need something in there to weigh it down, why not something that can house the bacteria? I rinsed out the bio cubes today, they had a lot more mulm than the sponge itself. Added three more- there was just enough space.