So, a little info before we get to it. I'm pretty sure this is all right, correct me if I'm wrong.
Humidity control changes with the seasons.
1- If you use an air-conditioner, your fishroom will be dry, they are just like a dehumidifier.
2- An exhaust fan will push your humid air outside, and bring dryer outside air back indoors through the cracks in your house.
3- A regular dehumidifier puts out a lot of heat, so it would be nice in the winter. But in the summer, an AC unit would be much nicer/more efficient.
4-Normally people don't have humidity issues in the summer, as the cooler indoor air can't hold as much moisture as the warm outdoor air.
5-So, the only time you really would want to run a regular dehumidifier would be in the winter. At which point, the one Calfo refers to is much more efficient. But, it is a
DIY.
An exhaust fan (many use a bathroom one) pushes your warm moist air out, in doing so pulls cold air in. This design exhausts your warm moist air, but keeps the heat inside. As the cold outdoor air passes the warm indoor air, the heat is transferred through the metal duct. You exaust your humidity, but keep your heat.

8" metal heating duct outside, 6" inside. Or, 6" and 4". Insulate the 8" duct.