Motionless air or water – for healthy cannabis growth, water must drain and not become stagnant as well as you need good air circulation.
Merriam-Webster Online Dictionary
stagnant (adjective)1.
a) not flowing in a current or stream - stagnant water
b) - stale long disuse had made the air stagnant and foul Bram Stoker
2.
not advancing or developing - a stagnant economy
inactive, motionless, still, sluggish, dormant