I would like to start the problem of getting air to the diffuser even just under the water's surface. Friction Loss in the lenght of tubing may be the problem in this one. In 1/2" tubing, typically required psi will be 0.16psi / 100' of tubing. Take that in combination of the depth, it takes 1psi to go down 27" of water and that will be your total psi needed. As Psi increases, cfm decreases. Therefore you may not have had enough volume of air for the combination of depth and lenght of air line.
Compressors; a new model is on the market now that can handle the psi and friction loss and provides 1.6 cfm at 20psi and is only 1/3hp. The name of the compressor is BrookWood.
Diffusers: You are correct that there are many different types. For uniformity, a stone or membrane are of preference. Stones typicall provide 3mm bubbles and membranes have been tested at 0.5mm bubbles. The smaller the bubble, the greater the surface area and the greater the circulation with smaller hp compressor.
Algae sitll present even with a diffuser: the total volume of the pond should be calculated and then back into how many diffusers of the type you are looking at using will be required to turnover the volume at least 0.75 times per day. There have been case studies done to determine this turnover rate as a goal to satisfy oxygen demand and allow for precipitation of iron and bonding of phosphates to the sediment layer thereby decreasing the food source for algae.
Summary: I would look into a compressor that will provide you enough psi and volume of air to reach the bottom of your pond, locate a quality diffuser, and if you fish in the pond, spend the money on the weighted tubing to prevent fishhook damage and kinking not to mention the time saved in not having to weight the standard garden hose to the bottom. Finally, make sure the diffuser system you will be using is sized properly for your pond. If you under aerate a pond, you will simply be causing yourself more headaches by increasing the nutrient amounts free-floating in the water column by suspending it from the bottom without satisfying the oxygen demand of the pond.
