Great information above @charliebankston. Beneficial bacteria need two things, fast water movement and media that the water can move through easily the media, not around or that can clog quickly. Any HOB
can be set up really well by just replacing everything inside with simple plastic pot scrubbies, as many that will fit. Clean quickly under water for about 5 seconds every 6+ months or more. All that brown 'gunk' is your BB that needs to be preserved to get clear water.