Purified water should be used for watering purposes, although 'hard water' is tolerated. This is done to minimise the build-up of minerals and chemicals in the soil. Water purity greater than 100 p.p.m. of total dissolved solids is often quoted as ideal. A reverse osmosis unit can be used to filter the water or, alternatively, bottled distilled water can be purchased. Watering should be done regularly. However, plants should not be allowed to sit in water, as this may lead to root rot.