Round blue porous stone

It connects to any air pump whose outlet takes a 4/6 mm pipe, the usual diameter for aquarium air pumps.

As long as the cloud of bubbles stays dense and even all the way round, the stone is diffusing properly. Once the bubbles grow larger, become spaced out and come from one side only, its pores have clogged.

Soaking in an acid bath reopens the pores once or twice, then the effect stops. On a part of this price and size, keeping a new stone in reserve lets you replace it in 30 seconds, instead of half a day of soaking.

You need one as soon as you connect several air stones to the same pump, otherwise the nearest takes most of the air. On a single diffuser, the valve is mainly used to calm the current in a planted tank or a fry tank.

Place it on the bottom, and preferably in a corner rather than under the filter outlet. The bubbles carry water from the bottom up to the surface, so a stone placed opposite the filter circulates the whole volume instead of reinforcing a current that already exists.

Yes, each one connects to its own line from a distribution valve, never in series. A series setup gives the last stone whatever is left, which is often very little.