Aquariums will increase humidity in a room (obviously), but quite honestly I don't know that it wil be enough to cause mold to grow in a basement if your basement doesn't already have mold problems. I use a de-humidifier in my basement at all times, so I don't experience any issues. I will say that the output of the de-humidifier seems to be affected far more by the central air than it does the fish tank. I get roughly the same quantity of water out of it that I always have with or without 115 gallon fish tank running. with my tank it goes some weeks with the glass top on and some with it off, evaporation is about 1 gallon a week when covered and 2 gallons when not. relatively small amounts of water for air in Ohio.
