Why is air movement important?
Very few people like stuffy rooms. Same goes for many plants -- though some plants do well in a stagnant environment with no air movement. For most plants air movement is important to maintain health. Stagnant air tends to lend itself to the growth of fungi and bacteria, certain types of which can harm/kill your plants.
In some terrs, particularly small ones [10 gal or less]it may be sufficient to leave the top off or cracked a bit. In larger terrs, it is often necessary to install some sort of small fan[s] in order to circulate the air. Research, research, research.