anything with an extended surface will do.
i've seen plastic straws used, and most plastic scrap will do a fine job.
but i resolve to lava rocks and other porous stone.
very cheap and it has a very large surface.
i just got some big lava rocks, and bashed them into smaller pieces and tossed them in the sump.
old t-shirts ripped to shreds will work fine too.
the blue filterfloss is one of my favorites too, and i allways use a 2 foot layer of that in my prefilter.
even if that gets dirty, it will take a long time until the flow is reduced by the dirt.
And dirty floss holds more dirt in the process.
if i was rich,or at least any richer than i am now i'd fill my sump up with ceramic rings, but i checked how much that would have cost me, and it's a small fortune.
the larger sumps for sveral 100gl+ tanks for lfs setups that i've seen were jam-packed with ceramic rings.
keep in mind, that any smoth surfaced plastics will have a very smooth surface, esp. made to hold as less as possible bacteria.
samewith any food grade disposable plastic, and medicinal use dispencers.
trust me, you can do better,i work with those plactis, and i know they are eto be as strerile as possible.
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