From an HVAC service tech. The air exchangers mentioned previously will work as advertised. Are they big enough for your app? maybe.
You give a size of 20x12 ft (240 square feet)
Depending on the insulation level of this steel structure and budget, you may be better off installing a small ductless hvac system similar to this.
http://mrslim.com/Products/Category.asp?ProductCategoryID=24
providing both heat in the winter and a/c in the summer.
If you only want dehumidification, most larger models sold at the big box stores have humidistats built in so you set the desired level just like the stat for your furnace a/c and it cycles accordingly to try and meet your needs.
Most are "portable", but I have seen them with permanent drain lines installed so you don't have to worry about emptying the water pan.
Most also give the recommended space size they can handle. For a fish room, double that recommendation.
As also noted, if you decrease the humidity, you will probably need more heat in the tank(s). Dry air has an affinity for water so more evaporation will take place. More evaporation will tend to have a cooling effect on the tank(s) and the space.
Your home a/c is really nothing more than a large dehumidifier/heat exchanger.
It makes us "feel" cool by removing moisture from the air. The air then removes moisture from us.
Believe it or not, I can make you hunt for a sweater in an 80 degree room, or sweat like a madman in a 65 degree room by manipulating humidity levels.
The mold/mildew issue should not be a problem until you get to around 70% or higher constantly.
Sorry for the length. hope it helps.