Young Macdonald's Farm Nitrate Nitrate, the final product of the bio filter, does not seem to cause many problems to the fish and is mostly managed by the daily replacement of water which is removed when cleaning settling tanks and evaporation. If from 5 to 10 % of the total water is replaced daily, it will help control nitrate levels. Nitrates may also be reduced by a denitrification process which simply involves moving some of the water from the system into an empty tank- stir the water to mix completely and then allow to sit for several days decent off the clear water top, discard the settled material and the decanted water may be returned to the system as the nitrate has been converted to atmospheric nitrogen and simply goes off into the air. NOTE: SEE - Remote water quality management