well sure I could change 50% a day and get water that doesn't smell. lets do the math here 110 gallons a day x 31 days 3410 gallons a month.
So 3410 gallons that needs 341 ml of prime, best possible price 12.00 already 7.00 more then the carbon, but sure its a waste of money to have that crazy carbon. Then we have to have water thats about 5 units of water which is about 45.00 so now we are at 57.00 wow 52 dollars more then the carbon.
Lets tone this down a bunch to 100% per week so 880 gallons thats 1 unit of water so about 8.00 but i normally do 50% a week so we will say thats 4.00 in water a month omg i am saving 1.00 so far over the carbon, oh wait no i am not i for got prime so thats 100ml needed best price 3.00 so now i am spending 2.00 more.
So as you can clearly see you are so right its such a huge waste of money. I knew this silly math would not help me in real life.
One more thing carbon helps with, airborne contaminates, how you say. Your tank has the smallest of smallest openings. What happens is water evaporates through this opening, so you have very humid air rising, then dust or other contaminates come in contact with this humid air, it becomes heavy and falls back into the tank. Or what about this your cleaning something with chemicals and omg you rares fish is getting sucked into the filter, do we have time to wash hands or do we not even consider that and rush to save our precious fish.