Cecil and Float, Yes the back pressure is the starting point on the calculations on a linear pump, This is why often Airstones vs rubber membrane diffusers are used with linear pumps in shallow water and often a coarse stone used vs a fine.(less back pressure) but less vertical current too. (not always good)The 4.2 CFM is open flow and calculate 1/2 lb of psi required per ft of water depth.Instead of trying to put a pencil to it I would run your depths and hose lengths past AES and they will give you the output and perhaps the turnover. I would assume you want to maintain 70-90% saturation in the summer months for your operation.If I ever need stones for slower turnover or a given situation I use Great lakes or sweetwater glass bonded silica stones from them.Hope I did not mislead, Ted
