HOW MUCH AIR DO I NEED TO RUN THE FOUR FILTERS
There are really two things you need from the air - pressure and volume
Volume per unit time or flow is often rated in cubic ft./min. You really don't need any more than you would for 8 airstones. Just a strong, steady stream of bubbles on each filter. Airpumps are often rated in # of outlets, which is a measure of air output or flow. But this # is often measured against no or little water pressure. Blowers often have great volume, but low pressure.
You need enough air pressure to overcome the weight of water at the bottom of your tanks so the air can push through. Ratings on size of tank or depth of tank attempt to convey the amount of air pressure a pump puts out. Air compressors have great pressure.
Of course the two variables are related and the graph of output (volume) is dependent on pressure. So you get fewer outlets in deep tanks. Some manufacturers will give you that graph, others just give their pump a number and let you figure empirically if it will work for you.
I like Jehmco's descriptions of air pumps. But all their pumps are big enough for an entire fish room.
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