A UV filter is essential to burn out single celled algae which gives pond water its murky, green appearance. The UV bulb will burn the algae, causing it to become sticky and thus it can be removed by the mechanical filtration. The bulb is changed after approx. 10,000 hours. We recommend you change it around Easter so that it is performing at its best through the bright Summer months. A general rule of thumb is to allow 10W per 1000gal (4500l) but if you pond does not benefit from shading or is shallower than 1m then you may need to increase this.