Its not as much about how many diffusers as it is how many times per 24 hours do you want / need to turn your water,once twice etc?If your looking at a 3 disc per station setup then be sure to get the test data that gives the lift at various depths and various cfm's. Find out the operating range of the station and stay away from the top end of the range. If you exceed the range with some membranes you can actually start to produce larger bubbles that lower your lifting capacity.Placement of the stations will then also come into play.If your carrying capacity is high and you have other factors occur like a bloom crash or sudden plant dieoff then yes you can have a kill even with a properly sized aeration system.Proper aeration will take out the daily highs and lows of your oxygen swing but cant compete with a sudden "demand" that is access of supply.