i don't think it goes bad. it's not like some of the other chemicals that have to be refrigerated after opening. those you can tell they've gone bad when you smell it and it smells like rotton eggs.:eek1: i dose like a capful of purple up every week or so into my 55g just to help maintain mag, calc, and the iodine for the softies. i don't think it does much and chances are i will never buy a bottle again after it is gone.