The present disclosure relates to an internal filter for freshwater aquariums, preferably up to 100L, more preferably between 40-80L, wherein the water circuit passing through the biological filtration is separated from the water circuit passing through the other filtrations (mechanical and chemical). Accordingly the filter has a body with a water inlet, a water outlet, a first pumped flow circuit for a mechanical treatment by a chemical treatment, a second flow circuit for a biological treatment, a separator for separating water flow from the inlet into two subflows for each of the first and second circuits, wherein the filter is configured for receiving a pump for pulling the first subflow from the chemical treatment circuit to the water outlet, wherein the outlet of the second circuit is connected to the inlet of the first circuit. This system can be applied in the small/medium size aquarium industry.
