If you see a white (bath-tub) ring about 50mm high around your fibreglass pool, don’t panic, especially if you have a floating pool blanket. It may look like it’s been bleached by the pool chlorine, but in most cases, it’s due to the pool blanket rubbing onto the fibreglass. As it rubs over the years, it slowly deposits a smooth layer of vinyl onto the pool wall surface. This layer or ring-mark will be slightly smoother than the wall in many cases. It will also generally look white as the blankets are usually a light, white-ish blue. Try a non-metallic pot scrubber and it should rub off with whitish particles dispersing in the water. Be careful not to scratch the gel-coat.