HNY, The bacteria grows inside where the bio balls(media) are, the coil is there to de-oxygen-nate the water to create the low ph(if I remember correctly) that the bacteria needs. Some ppl wrap the coil around the outside of the tube. Hope this helps, Luna _________ 9YR OLD 90G 30G w/d Mag18 150G skimmer 692w MH,yel&kole tang,foxface,midas&convict blenny,B&G chromies,Blk/yel fin chromie blackcap,nemo,neon goby,6line; Blastomussa Merleti,Acan ,BUBBLE,Torch,LTA, Goniopora(2), Acropora(2),Brain, Moon, assort zoas, yel& G star polyps, R&G open brain, P&B ricordia, montiporas, cup&candy corals, enias,B/G mush,flower ane(2), cherry red mussa, dusters,cleaning crew  |