David,Good question. One of the reasons the linear pumps are not that popular is the depth at which they operate.The AL Alita-80 for example has a suggested operating range of .05-.25 bar or 3.5 psi being .25 bar. Figure that each 1 ft of water depth that you place your diffuser in will have 1/2 lb of back pressure to overcome the downward force of the water. 12 ft equals 6 psi backpressue plus nothing calculated for line loss due to length and friction depending on hose ID.So if we use this pump at 6psi the cfm drop to approx 1.5 or 37% of the free flow.We are now way over the suggested operating range and will shorten pump life.This would be an expensive way to get 1.5 cfm. Your pond could take approx 3.5-4.5 cfm depending on where you set your goals and your stocking rate.Linear pumps are very good pumps for shallow ponds or water garden situations.Most rotary vanes (exception high pressure models) are limited to 10psi but that is 18-20 ft of water depth. Again not taking into consideration line loss due to total hose lengths. Piston pumps are now very popular since they are very price competitive to rotary vanes. The advantage of a piston is the ability to operate in water to 30-35 ft depth and very very long tubing runs for pond and lake sites that do not have electric power near by. An example of this is a 16 psi model Thomas WOB-L pump that has diffusers in 12 ft of water could have the pump located 7300 ft (1.38 miles)away and still be within the operating psi range of 15.9psi When a piston pump is located pond side or has short tubing runs of less than 1000ft it will run in that 6 psi range or 12-14 ft of water and runs very cool.This is why sizing a system includes knowing the desired turnover rate of the pond the depths,configuration and what the tested rates at different depths and cfm levels of the diffusers are.Ted