A cleansing system configured to receive aqueous medium from at least one fish tank and to return a cleaned aqueous medium to the at least one fish tank ; the cleansing system comprising: - a cleansing system inlet arranged at a cleansing system inlet level to inlet the aqueous medium into a first volume that is substantially surrounded by: - an annulus section having a second volume , and where the first volume and the second volume are in communication via at least a first-second-overflow leveller arranged at a first-second overflow leveller level and to radially overflow the aqueous medium from the first volume to the second volume ; wherein the annulus section is parted in arc-sections forming at least a further third volume in the annulus section , where the first volume and the third volume are in communication via at least a first-third-overflow leveller arranged at a first-third overflow leveller level and to radially overflow the aqueous medium from the first volume to the third volume ; and the second volume and the third volume are in communication via at least a second-third-overflow leveller arranged at a second-third overflow leveller level and to tangentially overflow the aqueous medium from the third volume to the second volume ; and - a cleansing system outlet arranged in the annulus section configured to outletting the now cleaned aqueous medium from the annulus section . - a cleansing system inlet arranged at a cleansing system inlet level to inlet the aqueous medium into a first volume that is substantially surrounded by: - an annulus section having a second volume , and where the first volume and the second volume are in communication via at least a first-second-overflow leveller arranged at a first-second overflow leveller level and to radially overflow the aqueous medium from the first volume to the second volume ; wherein the annulus section is parted in arc-sections forming at least a further third volume in the annulus section , where the first volume and the third volume are in communication via at least a first-third-overflow leveller arranged at a first-third overflow leveller level and to radially overflow the aqueous medium from the first volume to the third volume ; and the second volume and the third volume are in communication via at least a second-third-overflow leveller arranged at a second-third overflow leveller level and to tangentially overflow the aqueous medium from the third volume to the second volume ; and - a cleansing system outlet arranged in the annulus section configured to outletting the now cleaned aqueous medium from the annulus section . - an annulus section having a second volume , and where the first volume and the second volume are in communication via at least a first-second-overflow leveller arranged at a first-second overflow leveller level and to radially overflow the aqueous medium from the first volume to the second volume ; wherein the annulus section is parted in arc-sections forming at least a further third volume in the annulus section , where the first volume and the third volume are in communication via at least a first-third-overflow leveller arranged at a first-third overflow leveller level and to radially overflow the aqueous medium from the first volume to the third volume ; and the second volume and the third volume are in communication via at least a second-third-overflow leveller arranged at a second-third overflow leveller level and to tangentially overflow the aqueous medium from the third volume to the second volume ; and - a cleansing system outlet arranged in the annulus section configured to outletting the now cleaned aqueous medium from the annulus section . <EOS>