The hollow balls are intended to cover open water baths, if mounting a fix cover is not possible or disturbing. The balls are simply put into the bath where they align to an even layer. The water surface should be covered completely. The balls reduce: heat loss, evaporation loss (up to 90%), oxygen input, odors, action of light. Made of Polypropylene, diameter 20 mm, pack of 1000 pcs., only for water up to 100 °C.