For those looking for a solution beyond a floor fan...
Common molds typically grow at 55% humidity and above.
Humidistats work much like the wall thermostat in your house and are used in the greenhouse industry a lot. Instead of sensing temperature they sense relative humidity and they're common in airtight super insulated homes and other environments where you need to monitor the interior air to ensure that it's not getting too stale or too polluted. For those with outbuildings that are your fish rooms you could rig up an exhaust fan on an exterior wall and use a humidistat as your control set such that anytime the humidity reached 50% the fan would automatically come on and keep your tank room out of the danger zone.
Humidistats are available on Amazon in the ballpark of fifty US bucks. Depending on the size of your fish room you may be able to install it yourself and for less than $200 total. If you're unfamiliar w/ CFM ratings and the like there are I'm sure several here that can help size your exhaust fan.