If you keep the foam, clean it frequently. Filter bags are a very popular option for microbubble control as well as particulate filtration for those who use some type of prefilter. Most users have several or more so they can just swap them out 2 or 3 times a week and wash them when the dirty ones accumulate. Filter material that traps detritus can quickly become a nutrient sink with bacteria to convert through the cycle to nitrate but not beyond. So sponges and other filter materials not changed or cleaned frequently can contribute to high nitrate levels. I use large 100 micrcon "felt" filter socks/bags and change them out atleast 2 times per week. I have a couple dozen and I just wash them in the washer with bleach and a couple of extra rinse cycles when the dirty ones accumulate ... once a month - 6 weeks. Many filter sock users do the same.
Sponges, in my opinion, are a PITA because of all the pores within the sponge that can trap detritus and particulate matter deep in the sponge. Makes it very difficult or impossible to thoroughly clean.
