In an effort to please my wife I am taking an idea from our home grow friends. She hates the smell of fruit fly cultures, I mean hates them. Maybe it is a good thing, but she will not even go into the frog room.
Carbon air filters cost more than $100-$200 just for the filter and another $100+ for the fan. I came across a cheap alternative DIY. Here is where I got the idea.
The ultimate Odour control thread - Marijuana Growing
Step 1:
Get the parts.
Since my fan has a diameter of 4" this is what I used, adjust to the size of your fan.
1 - 4" coupling
1 - 4" cap
2 - 4" hose clamps
1 - 6" cap
1 - 6" to 4" reducer
2 - 6" hose clamps
1 - 10' x 3' -- 1/4" hardware cloth/wiring
1 - Box of sheet metal screws
1 - pantyhose, I used the opaque, non sheer.
poly "quilt filler" (from craft store)
Step 2:
Assemble the smaller/inside tube first. Use the cap and coupling to make a tube using the hardware cloth. Adjust the length to your needs.
I found it easiest to tie the middle (make sure the hardware cloth is lined up) with some wire temporary then use the hose clamps to install the cap and coupling. I wrapped the whole thing with some steel wire to keep the hardware cloth together. I then put on the pantyhose. On the tube not my legs.
Be careful not to snag them on the hardware cloth. Loosened up the hose clamps and slid the pantyhose under and re-secured them.
The pantyhose will keep the carbon from coming through the hardware cloth.
Step 3:
Slide the 4" coupling into the 6" side of the reducer.
You will want to put some of the poly quilting batting in here to block any small pieces of carbon from getting through, although it should be a fairly tight fit.
Step 4:
Using the reducer and the 6" cap make another tube using the hardware cloth keeping the smaller tube you made inside. You will have to adjust it so the cap from the inside tube will fit snug against the cap of the 6" tube for securement later.
Step 5:
Sorry no pics yet as I have not gotten this far myself. Waiting on carbon. Wrap the outer tube in the pantyhose and then in poly quilting batting. The quilting batting will act as a dust prefilter.
Step 6:
Remove the 6" cap and pour in carbon, shake it down, then pour in more until the 1" gap between the inner and outer tube is full. Replace cap and secure every thing with sheet metal screws.
Step 7:
Install fan so it blows out, sucking in air through the poly then through the carbon.
I got this fan from a garage sale for $5. It is normally used for an air duct booster. Although I would bet a computer fan would work just as well.
If done right, you could use this to vent a room and out going air will be scent free. If not you may be drawing in air too fast for the carbon to absorb odors. I will use it to polish off the room allowing the filter to run and continuously clean the air. We will see how well it works this way.
As far as the carbon, I will be getting mine from Bulk Reef Supply. It is a local company and seems to have the best price.
15 Pounds - 5 Gallons Bulk Lignite Aquarium Carbon - English
I will post more pics when I get it fully set up.
Total cost about $30, not including carbon or the fan.
DIY Carbon air and odor filter
Coming up with a solution for a problem. Now in my college years, I would have had a different answer. 
Grow rooms are grow rooms, whether it is frogs and bromeliads or medicinal plants. They have a lot of great ideas that can be transfered to our hobby.
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