Seachem Tidal FiltersVentilation slots Ventilation slots in the lid of the filter and aeration grooves in the outflow ensure that water returned to the tank is fully oxygenated and maximize gas exchange during filtration. Bottom to top flow The pump pushes water up into the basket of the filter, entering through the bottom of the basket and being pushed out the top. A bottom to top flow ensures that water comes into contact will all filter media, and cannot go around or