Jeff-
I've got some concerns for your build long run; you voiced two things that scare me greatly....
Tying into a vent line is not only not good; as voiced above it can be deadly for you and your family. I'd strongly recommend you cut a hole and direct vent your fishtank room (As mine currently is, through a 4" dryer-vent style hole.)
Next, although your humidity in the room will be lower than say, after you took a long steamy shower with the room all closed up, it'll be continuous. Plus, the salt in the air is REALLY corrosive. I ran a utility room behind my tank with a fresh air return line in it for about 5 years; ask me what that vent looked like (And that was with a dehumidifier.) Using Green or Purple boards (Sheetrock and USG brands, respectively, for moisture resistance) should be, without question, used. Ditto for fiberglass tape for seaming.
Is your furnace room also the room where your breaker box is located? If so, I'd keep it out of that room. Corrosive elements plus your electric supply can also be deadly (Think heat/fire from resistance.) Keeping it next to a furnace will degrade and shorten the life of the PC Boards internally (They're usually not potted/sealed; most aren't), the fan, and the duct work.
Lastly, adding plastic to exterior facing walls is actually code here (8 mil; plus taping where there are staple holes/seams for vapor barrier.) Adding it to internal walls can have odd effects for moisture and heat dissipation, as well as airflow between rooms.
Keep in mind, building codes vary from location to location, but should be viewed as *minimum* safe levels to build to, not as something you should strive for. Contractors in housing developments build homes for other people, not themselves, and will build as cheaply as they can legally get away with, including saving $2 a sheet on drywall, as long as it fails after the warranty period.
I'd seriously suggest consulting a contractor; even for ideas. Period. If you're not familiar with the effects of airborne corrosives in your house, and/or basic local code or building materials. Some will offer a visit for "consulting" on your home projects. It may save your life, and it'll definitely save your wallet in the long run.
If you ask how I know, I flip, and rebuild higher-end real estate (I'm on house 12; never had issues passing a code inspection post and during rebuild, either.)
You *can* get away without consulting a pro, but I'm a firm believer in spending a little more upfront and making sure it doesn't fail in the long run (Or try to kill you.)