Available in two popular sizes, 250 mesh (65 micron opening) and 150 mesh (105 micron opening), these are professional quality, double-seamed nylon mesh bags, approximately 20 cm. x 30 cm. (8" x 10"). Mesh bags are indispensable around the hatchery or fish room. The 250 mesh size is suitable for harvesting rotifers and other small zooplankton and phytoplankton. The 150 mesh size is used for artemia, daphnia and other larger zooplankton. In addition to their obvious use as nets, these Mesh Bags are used as water filters, either to prevent the introduction of unwanted species from natural water sources, or; to cover a standing drain to prevent desirable species from escaping. Mesh Bags can also be used as a "tea bag" to reduce food particle size for fry; because they're resistant to most chemicals, they can also be used when decapsulating, rinsing or disinfecting artemia cysts. Extremely versatile. I even have one that I use for dry-hopping when brewing IPA's!