Here is the excerpt of an email that I just received:
I am currently in Vietnam, and visiting villages that recovers rainwater (because no running water) I asked them how were they doing for fight against the mosquitoes which must have proliferated in their jars (because despite the fact that I saw very many full jars, I did not see any larvae), and they answered me that they put "stone of 'alum"...
Has anyone ever heard of this use?
Someone tested? (I have alum stones but I lack stagnant water with mosquito larvae ...)