Filter unit for aquariums (1), which allows a high efficiency of chemical-biological filtering with reduced times of recirculation of the water, comprising: a container (10), provided with a fluid inlet (11) and a fluid outlet (12) to allow the fluid communication thereof with an aquarium tank pumping means intended for the circulation of a liquid flow inside said container (10) between said fluid inlet (11) and said fluid outlet (12) along at least one main path (100) a first filter (30), arranged to intercept all of the liquid flow which follows said main path (100), performing on it at least one mechanical filtering action at least one second filter (40) arranged to perform at least one filtering action other than the mechanical one said main path (100) dividing into at least one first slow subpath (100b) and a fast subpath (100a), said second filter (40) being arranged to intercept, slowing it down, only the part of the liquid flow which follows the first slow subpath (100b), the part of liquid flow which follows the fast subpath (100a) bypassing said second filter (40) without being slowed down by it.
