03-20-2013, 07:41 AM
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Most likely biological. Keep in mind that bioballs are probably the most efficient method of aerobic biofiltration available to the common hobbyist, so bacteria will grow much more rapidly than in let's say a canister filter. Bacteria will create a biofilm to help trap nutrients, which I've seen in many shapes and forms (I've had 2" PVC clog on me with what I can only describe as bacterial snot lol). Just based on my research in biological water treatment, that certainly looks like biofilm it to me.
Last edited by funkman262; 03-20-2013 at 07:49 AM..