A filter apparatus (1) for mechanically filtering a liquid, such as water (W). The filter apparatus (1) includes a filter chamber (5) for containing a plurality of filter elements (7) that form a filter pack (29) for filtering the liquid. The filter chamber (5) has a liquid inlet (12) and a liquid outlet (14). The filter apparatus (1) includes means for introducing air into the filter chamber (5) through one or more air inlets (25) to agitate the filter elements (7). The filter chamber (5) is substantially sealed and the air introducing means (22) may be configured to draw air into the filter chamber (5) as liquid is drained from said filter chamber (5). The filter chamber (5) may have a substantially constant profile or a substantially continuously tapered profile along its longitudinal axis (X-X). Further inventions are directed to a filter apparatus characterised by being configured to generate a flow of liquid, an aquatic system characterised by a container adapted to contain a predetermined volume of liquid, a method of filtering a liquid characterised by filter elements having an open cell structure and method of mechanically filtering a liquid characterised by backwashing filter apparatus. The filter apparatus may be used to filter water in aquariums and pools.
