@Le 3. New strainers also filter sand and gravel should be placed in the filter as the present strainers and sand and gravel are in very bad shape. In addition, before placing in the new strainers and sand and gravel, the inside of the filter should be brushed down by means of a wire brush or other suitable means for removing as much of the scale as possible. Also, the agitator on the present filter should be removed, at least that portion inside the filter. 4.