I use a closed jar (with an airlock) for all my ferments. This includes water kefir, milk kefir, and sourdough. When any kefir is made aerobically, there are unwanted wild yeasts and bacteria introduced into the culture that lead to off flavors and sometimes textures as well. It also encourages the growth of kahm yeast, mold and excess histamine. My son is deathly allergic to ingesting mold so I do not take any chances there, at all. In addition, although kahm is considered "safe for consumption," I do not like the look, taste or texture it imparts and dump anything that gets kahm. I ferment anaerobically as I like my ferments to be pure, taste great, and have vibrant color. My favorite fermentation vessel is the Pickl-It. I've used several others and always come back to this one. I've been fermenting for 8 years now and after millions of batches of ferments my love is unconditional. Buy small jars or call and order lids only to save money. You can get the Fido jars inexpensively at places like Home Goods, TJ Maxx, Ross, Marshall's, and Sur La Table. I have a huge collection of 1/2L-5L jars I've collected from these stores. Pickl-It does not have an affiliate program that I'm aware of, and anything you purchase from them does not benefit me financially. I just truly love their products (and the creator, Kathleen, was sent from Heaven!).