<font size="3"><span style="font-family: times new roman,times,serif;">"</span> <font style="font-family: times new roman,times,serif;" size="3">Because the water flows through the denitrator very slowly, it is in contact for a long time with that bacterial bed, and is cleansed in the process of nitrates and some other compounds. To keep the bed active, and to make the chemical and biological processes that occur in the denitrator complete, a carbon source needs to be added to the filter as well. Lactose, mixes of lactose and other sugars, for example fructose, are used to achieve the desired result. <font size="3"><span style="font-family: times new roman,times,serif;">In certain filters alcohol is used instead of sugars, and is placed in bags with micron pores to make it migrate very slowly into the water. The need to add a high quality nutrient is not to be underestimated. Indeed, it will make the difference between a filter that is rather easy to operate and regulate, and one that is giving you constant problems."</span> <font size="3"> http://www.netpets.org/fish/reference/thielbook/thielbook3l. html