Here is a nifty innovation I've not heard about before. These are the valves controlling the air pressure to the diffuser drains. If a valve is not fully open, there is back pressure on the air pump which can reduce its useful life. By adding a valved relief outlet, the excess air pressure is simply dispersed into the air. This causes an irritating noise. So, airstones are attached and simply left hanging. They act as mufflers. Cool, Carl.
