The reason for this was the high flow-through rate at 200% a day, which did not allow nitrifying bacteria to develop effectively on the sand substrate initially. Possibly the sand substrate in the flow-through water system started to function as an internal bio-filter after the first two months. (2005) suggested to periodically add sodium carbonate to the recirculation tank, therefore it would be interesting to test a recirculation system at lower flow-through rates for future research work instead of the high rates (200% a day) used in the present study.