You can see that one problem I had was in cleaning the filter sponge. I soaked it in an Excel/water mix, used a water hose on it, scrubbed it with my hands, pounded on it, let it dry in the sun, soaked it again and repeated trying to scrub it. And, it still has dead BBA on the surface, plus bits of sand in some pores. I'm hoping when I add fish they will help pick off the bits of dead algae.
I had this in operation for a year, with no cleaning, so it was pretty well loaded when I started. Lots of fine brown silt, much of which was probably the bacteria colony that had grown. Now it has to cycle all over again.