<p>The fish tank has a water treatment shaft (12) in which are situated a flooded fixed-bed reactor (68) for microbiological water treatment and a gas-introduction part (72) arranged underneath. Oxygen-containing gas is fed through the gas-introduction part (72) and the oxygen-enriched water is transported by the airlift pump effect in ascending flow through the fixed-bed reactor (68). A denitrification and sedimentation zone (104) is formed beneath the gas-introduction part (72). To form a mixing zone (76), the gas-introduction part (72) is arranged at a distance from the fixed-bed reactor (68) which lies submerged beneath the water surface (98) so that above this are situated a degassing and run-off zone (100). <IMAGE></p>
