There are times when aeratiing an earth bottom pond, retention pond, koi pond, water garden, or other body of water when it's necessary to deliver more air to one specific diffuser/airstone than other disffusers/airstones. More importantly, air is like water - it will find the path of least resistance. Therefore, airstones/diffusers with the shortest hose run will get more air than the diffusers/airstones further along in the system. That's why it's imperative to have control valves when there are more than 1 airstones/diffusers in the system.
