Its probably from the tang mix ; )

Your water crystals should keep for a long time if only mixed with water and kept in a clean container. I've kept the stuff for months in large glass and plastic containers, and regularly keep a small supply in a 6qt plastic 'shoebox' storage container for ease of serving to all of the colonies. Make sure the container is completely clean and you only mix with water - the roaches will suck it up whether its flavored or not = )