There are a number of things to look at.
1. Moisture: Affects the media and lowers its capacity. You first batch may have dropped in capacity as it was pulling out to much moisture from the air ( but I do not buy into this). This should be less on the second batch refill. Basements are more of a good thing as they usually have higher humidity. Getting dried out, from storage or when used, as the air passes over it in the CO2 unit is the biggest concern as the media is suppose to have moisture to operate. Most are around 10 - 20 % moisture. Many companies have give a rather large range for their product, i.e., 12 - 18 % on at data sheet. Media moisture should be ~ 18 % and during operation the moisture should never get below 2 %. When I see these CO2 media with zeolites or adding water to the bottom of the canister it tells me one thing, the media is low in moisture to begin with.
2. Shape: Different shapes affect the adsorption capacity. It has been shown the cylinders are better than spherical or granular. Even manufactures that sell different grain shapes tell you that.
3. Grain size: This can get tricky as one does not what it to small or to big. Grains in the 8-12 ( 2.5 - 5 mm) mesh will last ~ 1.3 times longer than 4 - 8 mesh ( 1.0 - 2.5) or about 4 % more adsorption. But this is for re-breathers and IMHO 4-8 mesh is better for us. One issue here is the smaller the GS the greater the pressure drop. Something you do not want.
4. Grain porosity: Some media are more pours which gives more surface area and more or quicker adsorption.
5. Dust and Harness : You want about the lowest dust and highest hardness you can get.
For all practical purposes most are about same, i.e., there are no leaps and bound differences. Any media would have lost adsorption capacity in the first use if there are humidly problems in the basement.
The media sold by thefilterguys is about the best there its. It is # 1 in hospitals and re-breathers.
I'm one that does not believe in claims unless it is backed up by a data sheet. I have seen TFG data sheet on grain size/shape, moisture, adsorption capacity,etc.
