Laboratory bath accessories are used to extend the capabilities of water and circulating water baths. Though baths are usually filled with tap or distilled water, some applications require the use of bath fluids including fluid ethylene glycol, liquid silicone, or an algae-control agent. Sometimes a water bath rack or shelf is customized to hold flasks, test tubes, or other specimen containers most often processed in the bath. An assay plate can be used to hold several small samples to ensure consistent testing. Bath covers vary in shape and material. A gabled cover allows the steam to run down the cover to the sides of the bath, reducing the risk of contamination, and clear lids allow the operator to observe specimens without the need to open the bath. Polypropylene balls can also be used in place of a cover, allowing operators to use the bath for tall specimen containers.