Kind of off topic, but i have found that what I have added for a completely different reason, has resolved my humidity issues.
I installed a quietcool whole house fan for this summer. I will spare you the details of my HUGE $$ savings by automating this to run in lieu of my A/C unit, but the benefit from running this thing has greatly reduced my house's humidity and also substantially freshened the air in the house.
I know you are considering a vent fan, but I'm just throwing this out there as a possible solution for multiple issues.
My setup: Whole house fan running on Insteon receivers. Insteon thermostat with humidity sensor. Insteon Wireless thermostat to detect outside temp. All controlled my an ISY.
Short sdescription:
If outside temp is low, and inside temperature is higher and not at a set threshold, my ISY pauses my thermostat (A/C) and kicks on my whole house fan and relays to my fresh air inlet box. When threshold is met, cycle reverses.
I also have it kick on for a minute or so every hour to freshen the air up a bit. No major temperature change, but good fresh air and lower humidity.
Just a thought.
If your fish room is sealed, a whole house solution may not be for you. Inline fan, would be the way to go. Like others have said, you need a fresh air inlet (of some sort). Otherwise you fan will get loud and will not be effective due to its inability to remove air.