Microbial growth is a problem that I have been experimenting with. I tried adding a drop of nascent iodine tincture to my mix and it works really well. Silver hydrosol works too but is pricey . . . I am going to experiment with potassium iodide (KI) next since you can use it to sanitize water, as well as for that pesky, occasional, nuclear fallout emergency I happen to have it laying around for, it should work well . . .