Toilet tank/reservoir mold / mildew problem

I have a rental house which just became vacant. I notice in the toilet tanks, there is a mold / mildew problem. I'm not sure why since the house a/c works fine. Anyway, what is the best way to clean this inside the tank? I'm thinking of spraying bleach but I'm a little afraid if that will hurt the plastic guts or rubber seals inside the tank. Suggestions? Also any idea how to prevent this from happening again over time?
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Observer wrote:

I would pour a cup or two of bleach into the tank and scrub the inside with the toilet brush. It should keep the mildew down for quite a while. Mildew is probably as likely to erode the rubber seals as bleach might; I wouldn't worry about it. If it is only a very light amount of mildew, I probably wouldn't bother...I've never noticed mildew in my toilet tank.
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ttshell had written this in response to http://www.thestuccocompany.com/maintenance/Toilet-tank-reservoir-mold-mildew-problem-381800-.htm :
Observer wrote:

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I've found a combination of liquid bleach, hydrogen peroxide, oxi powder and baking powder has gotten mold and mildew out of the pots that are installed in the <a rel="follow" href="http://www.mesawasteservices.com/restroom_trailer.php ">toilet trailers</a> that I lease out to workcamps for yearly durations. You neverknow how those bad boys come back. Sometimes gawd awful.
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