Just bought 4 of these. So if i fill them with city chlorinated water, do i still need to add bleach? Or do i just need to recycle/refresh them every 6 months or so? If i need to add bleach, in what ratio? 5-8 drops per gallon? I'm sure this has been asked about a billion times before but a search here on this site results in way too many slightly related threads. Thanks.