When you are wanting to add air to a number of aquarium tanks there are a couple of ways of doing it with large air pumps.
The usual aquarium airpumps will have outlets of 1,2,4,6 and even 8 but if you have more devices you how do you do it with out using a bunch of splitters.
You can get large high flow air pumps that are designed for connecting 100 plus devices.
The way you choose to connect these pumps will also give you various results. If you have a group of devices or tanks in close proximity of each other you can usually use a manifold. A manifold will allow you to run air line direct from the airpump to the devices but keep in mind this may not be the best way of doing it.
When connecting manifolds you use 4mm airline and if all the devices are close that's great but if you have to run meters and meters of airline the pump has to work harder to push air through smaller tubing over larger distances which is not ideal.
The alternative is using a loop and this can be done with PVC tubing or even with irrigation hosing. By using irrigation or pvc and connecting it into a loop or figure 8 provides even flow of air throughout the loop. You can then use airline to go from the loop directly to the device or aquarium and this method is more efficient than a manifold and in 90% of the cases allow you to connect more devices or tanks.
Back pressure - back pressure is when the amount of air being produced is not being released and this pressure creates issues making the high flow air pump work harder, heating up - reducing the lifespan of not only the unit but also the diaphragms. You can reduce back pressure by simply allowing the built up air to be released and you can do this by opening up a airline or two but be careful as too much will result in not enough going to the devices and tanks so keep the balance..
The most common reason why there is not enough air is generally a simple one.
How to pick the right size is a common question and it is difficult to give a definitive answer. To use as a guide when using the loop on a 30cm deep tank to a single airstone or sponge you will generally want 1 litre per minute - what is 1 litre per minute? - we have a youtube at the bottom showing what 1 litre per minute is. If you want to use airbars then you will need more air delivered to them and you will not be able to connect as many and similarly the longer the lengths of airline and the deeper the tanks the more the units have to push will also reduce the amount of tanks and devices that can be connected.