Biological filtration is a method that uses naturally occurring nitrifying bacteria to detoxify nitrogenous wastes. One type of bacteria (Nitrosomonas spp.) converts ammonia to nitrite while another naturally occurring bacteria (Nitrobacter spp.)e nitrite into a far less harmful form of nitrogen wast converts the: nitrate. Nitrate is the final stage of the nitrify. Many invertebrates, unlike fish, are not tolerable to high nitrate concentrations. As a result, it is extremely important to make sure that nitrate levels do not reach high levels. In freshwater aquaria the optimal level of Nitrate should be below 20 mg/lt. Values of over 75 mg/lt should be avoided at all costs. In marine aquarium their level should not exceed 20 mg/l. A level of 5-10 mg/lt is the maximum for reef aquariums.The NO3 reagent from ELOS, especially developed for fresh water and sea water aquarium, allows the determination of the Nitrate level with the help of a finely tuned colour scale which shows a range of 5 – 75 mg/Lt
