Josh,
In new construction, houses are sealed much better than in older homes, including around the windows. I would expect higher humidity in a new home than in an older home. It would have nothing to do with window installment. The better the window installment, the fewer air leaks you are going to have, and the higher humidity you will experience.
Personally, I think the window guy is right that the humidity from your extensive fishroom might be contributing to the higher humidity in your home. When you get your new gadget, set it up in your fishroom to get a reading there. I'm going to guess that it will be quite high
That humdity will travel throughout your house, I'm afraid
If the source of your humidity is from your fishroom, you can turn off or turn down the humidifier on your furnace to counteract the humidity coming from your fishroom.